diff --git a/.prettierignore b/.prettierignore
new file mode 100644
index 0000000..23d67fc
--- /dev/null
+++ b/.prettierignore
@@ -0,0 +1,2 @@
+node_modules/
+yarn.lock
diff --git a/.prettierrc b/.prettierrc
new file mode 100644
index 0000000..8af31cd
--- /dev/null
+++ b/.prettierrc
@@ -0,0 +1,4 @@
+{
+    "tabWidth": 4,
+    "proseWrap": "always"
+}
diff --git a/README.md b/README.md
index 3888c6e..b06f8ae 100644
--- a/README.md
+++ b/README.md
@@ -1,28 +1,26 @@
 # bsdaily
 
-[bsdaily](https://git.eeqj.de/sneak/bsdaily) is a command-line utility
-written in [Go](https://golang.org) that carves a single day (or a range of
-days) of [Bluesky](https://bsky.app) firehose data out of a large,
-continuously-growing SQLite database and writes it out as a self-contained,
+[bsdaily](https://git.eeqj.de/sneak/bsdaily) is a command-line utility written
+in [Go](https://golang.org) that carves a single day (or a range of days) of
+[Bluesky](https://bsky.app) firehose data out of a large, continuously-growing
+SQLite database and writes it out as a self-contained,
 [zstd](https://facebook.github.io/zstd/)-compressed SQL dump. The dumps are
-named by date (e.g. `2026-06-27.sql.zst`), organized into per-month
-directories, and are designed to be published, archived, mirrored, and later
-re-merged back into a single database.
+named by date (e.g. `2026-06-27.sql.zst`), organized into per-month directories,
+and are designed to be published, archived, mirrored, and later re-merged back
+into a single database.
 
 The source database is read from a read-only [ZFS](https://openzfs.org)
-snapshot, so extraction never contends with the live firehose ingester that
-is writing to the original database. The tool is operationally
-conservative: it checks free disk space before starting, copies the snapshot
-to fast scratch storage, processes one day at a time to avoid SQLite lock
-contention, verifies every compressed output before publishing it, and
-writes output atomically via a temp-file-and-rename so a partial run never
-leaves a corrupt `.sql.zst` behind.
+snapshot, so extraction never contends with the live firehose ingester that is
+writing to the original database. The tool is operationally conservative: it
+checks free disk space before starting, copies the snapshot to fast scratch
+storage, processes one day at a time to avoid SQLite lock contention, verifies
+every compressed output before publishing it, and writes output atomically via a
+temp-file-and-rename so a partial run never leaves a corrupt `.sql.zst` behind.
 
-This project was written by [@sneak](https://sneak.berlin) to produce a
-daily, mergeable, publicly-mirrorable archive of the Bluesky firehose. It is
-currently a one-person effort. The current version is pre-1.0 and there has
-not yet been a versioned release; [SemVer](https://semver.org) will be used
-for releases.
+This project was written by [@sneak](https://sneak.berlin) to produce a daily,
+mergeable, publicly-mirrorable archive of the Bluesky firehose. It is currently
+a one-person effort. The current version is pre-1.0 and there has not yet been a
+versioned release; [SemVer](https://semver.org) will be used for releases.
 
 # Build Status
 
@@ -33,14 +31,14 @@ branch must always be green.
 
 Primary development happens on a privately-run Gitea instance at
 [https://git.eeqj.de/sneak/bsdaily](https://git.eeqj.de/sneak/bsdaily) and
-issues are [tracked
-there](https://git.eeqj.de/sneak/bsdaily/issues).
+issues are [tracked there](https://git.eeqj.de/sneak/bsdaily/issues).
 
-Changes must always be formatted with a standard `go fmt`, syntactically
-valid, and must pass the linting defined in the repository (presently the
-`golangci-lint` defaults), which can be run with a `make lint`. The `main`
-branch is protected and all changes must be made via [pull
-requests](https://git.eeqj.de/sneak/bsdaily/pulls) and pass CI to be merged.
+Changes must always be formatted with `make fmt` (`go fmt` for Go, prettier for
+Markdown), syntactically valid, and must pass the linting defined in the
+repository (presently the `golangci-lint` defaults), which can be run with a
+`make lint`. The `main` branch is protected and all changes must be made via
+[pull requests](https://git.eeqj.de/sneak/bsdaily/pulls) and pass CI to be
+merged.
 
 See [`REPO_POLICIES.md`](REPO_POLICIES.md) for detailed coding standards,
 tooling requirements, and workflow conventions.
@@ -50,55 +48,55 @@ tooling requirements, and workflow conventions.
 This repository adheres to the
 [Scripts to Rule Them All](https://github.com/github/scripts-to-rule-them-all)
 standard: normalized scripts in `script/` are the entrypoints for the
-development workflow, and the Makefile targets are thin shims that call
-them. We provide:
+development workflow, and the Makefile targets are thin shims that call them. We
+provide:
 
-- `script/bootstrap` — install all development dependencies (go, Go
-  module download); the linter is not installed on the host
+- `script/bootstrap` — install all development dependencies (go, Go module
+  download, node and yarn at pinned versions when absent, and the prettier
+  pinned in `package.json` and `yarn.lock`); a node or yarn already installed is
+  used whatever its version; the linter is not installed on the host
 - `script/setup` — make a fresh clone ready for development: runs
   `script/bootstrap`, then `script/install-precommit`
-- `script/projectname` — print the project name (used for the Docker
-  image tags)
-- `script/test` — build the Dockerfile's `test` phase, which runs the
-  test suite with `-race` (verbose rerun on failure)
+- `script/projectname` — print the project name (used for the Docker image tags)
+- `script/test` — build the Dockerfile's `test` phase, which runs the test suite
+  with `-race` (verbose rerun on failure)
 - `script/lint` — build the Dockerfile's `lint` phase, which runs
   `golangci-lint run ./...`
-- `script/fmt` — format the Go code with `go fmt` (writes)
-- `script/fmt-check` — check the Go formatting with `gofmt` (read-only)
-- `script/check` — run `script/test`, `script/lint`, and
-  `script/fmt-check`
-- `script/docker` — build the Docker image tagged via
-  `script/projectname`; the build runs the `lint` and `test` phases
-  first
-- `script/cibuild` — CI entrypoint: runs `script/bootstrap` and
-  `script/check`, then builds the Docker image tagged via
-  `script/projectname`
-- `script/precommit` — pre-commit gate: `go mod tidy` (must not change
-  `go.mod` or `go.sum`) and `go fmt`, then `script/check`
-- `script/install-precommit` — install the git pre-commit hook that
-  runs `script/precommit`
+- `script/fmt` — format the Go code with `go fmt` and every Markdown file with
+  prettier (writes)
+- `script/fmt-check` — check the Go formatting with `gofmt` and the Markdown
+  formatting with prettier (read-only)
+- `script/check` — run `script/test`, `script/lint`, and `script/fmt-check`
+- `script/docker` — build the Docker image tagged via `script/projectname`; the
+  build runs the `lint` and `test` phases first
+- `script/cibuild` — CI entrypoint: runs `script/bootstrap` and `script/check`,
+  then builds the Docker image tagged via `script/projectname`
+- `script/precommit` — pre-commit gate: `go mod tidy` (must not change `go.mod`
+  or `go.sum`) and `go fmt`, then `script/check`
+- `script/install-precommit` — install the git pre-commit hook that runs
+  `script/precommit`
 
-Every Docker build in `script/` is uncached, so the `lint` and `test`
-phases always run rather than being served from the build cache.
+Every Docker build in `script/` is uncached, so the `lint` and `test` phases
+always run rather than being served from the build cache.
 
 # Problem Statement
 
-A Bluesky firehose ingester writes every observed post (and associated
-users, hashtags, URLs, and media references) into a single ever-growing
-SQLite database, `firehose.db`. This database has several properties that
-make it awkward to publish or archive directly:
+A Bluesky firehose ingester writes every observed post (and associated users,
+hashtags, URLs, and media references) into a single ever-growing SQLite
+database, `firehose.db`. This database has several properties that make it
+awkward to publish or archive directly:
 
-- It is **large and always growing**, so re-publishing the whole thing every
-  day is wasteful.
+- It is **large and always growing**, so re-publishing the whole thing every day
+  is wasteful.
 - It is **continuously written**, so reading from it directly risks lock
   contention with the live ingester and inconsistent reads.
-- It is **monolithic**, so there is no natural unit at which to mirror,
-  share, or distribute "just yesterday's posts".
+- It is **monolithic**, so there is no natural unit at which to mirror, share,
+  or distribute "just yesterday's posts".
 
-What is wanted instead is a stable, immutable, per-day artifact: a small
-file containing exactly one calendar day of firehose data, cheap to publish,
-cheap to mirror, and trivially re-mergeable into a full database by anyone
-who collects a set of them.
+What is wanted instead is a stable, immutable, per-day artifact: a small file
+containing exactly one calendar day of firehose data, cheap to publish, cheap to
+mirror, and trivially re-mergeable into a full database by anyone who collects a
+set of them.
 
 # Proposed Solution
 
@@ -108,81 +106,81 @@ A tool, `bsdaily`, that:
   filesystem, so it reads from a consistent point-in-time copy that the live
   ingester cannot be writing to;
 - copies the snapshot's database files to fast scratch storage;
-- **extracts** a single day's `posts` (and all rows reachable from them) into
-  a fresh, minimal per-day SQLite database;
-- **dumps** that per-day database to SQL and pipes it through multithreaded
-  zstd compression;
-- **verifies** the compressed output (zstd integrity check plus a sanity
-  check that the decompressed stream actually looks like SQL);
+- **extracts** a single day's `posts` (and all rows reachable from them) into a
+  fresh, minimal per-day SQLite database;
+- **dumps** that per-day database to SQL and pipes it through multithreaded zstd
+  compression;
+- **verifies** the compressed output (zstd integrity check plus a sanity check
+  that the decompressed stream actually looks like SQL);
 - **publishes** the result atomically as
   `DailiesBase/YYYY-MM/YYYY-MM-DD.sql.zst`.
 
 Each daily dump is emitted with `INSERT` statements over the full schema
-(including the deduplicated `users`, `hashtags`, and `urls` lookup tables),
-so any collection of daily dumps can be merged into a single database by
-rewriting `INSERT INTO` to `INSERT OR IGNORE INTO` and replaying them in
-sequence. Two helper scripts ([`merge_daily_dumps.sh`](merge_daily_dumps.sh)
-and [`regenerate_auxiliary_tables.sql`](regenerate_auxiliary_tables.sql)) are
+(including the deduplicated `users`, `hashtags`, and `urls` lookup tables), so
+any collection of daily dumps can be merged into a single database by rewriting
+`INSERT INTO` to `INSERT OR IGNORE INTO` and replaying them in sequence. Two
+helper scripts ([`merge_daily_dumps.sh`](merge_daily_dumps.sh) and
+[`regenerate_auxiliary_tables.sql`](regenerate_auxiliary_tables.sql)) are
 included to do exactly this and to rebuild the aggregate statistics
 (`use_count`, `first_seen`, user `resolved_at`/`updated_at`) afterward.
 
 # Design Goals
 
-- **Never disturb the live ingester.** All reads come from a ZFS snapshot,
-  never the live database.
+- **Never disturb the live ingester.** All reads come from a ZFS snapshot, never
+  the live database.
 - **Crash-safe, idempotent runs.** Output is written to a temp file and
   atomically renamed; a day whose final output already exists is skipped, so
   re-running a range is safe and resumable.
-- **Mergeable output.** Daily dumps re-combine losslessly into a full
-  database via `INSERT OR IGNORE`.
+- **Mergeable output.** Daily dumps re-combine losslessly into a full database
+  via `INSERT OR IGNORE`.
 - **Operationally cautious.** Free-space preflight checks on both scratch and
   output filesystems; explicit verification of every artifact before it is
   published.
 - **Fast where it's free.** Large snapshot copies use a 256MiB buffer,
   pre-allocate the destination, and (on Linux) issue `posix_fadvise`
-  sequential/willneed hints; extraction uses aggressive,
-  crash-unsafe-by-design SQLite pragmas because the working data lives only
-  in disposable scratch space.
+  sequential/willneed hints; extraction uses aggressive, crash-unsafe-by-design
+  SQLite pragmas because the working data lives only in disposable scratch
+  space.
 
 # Non-Goals
 
-- **Real-time export.** `bsdaily` operates on daily snapshots; the freshest
-  day it can produce is the snapshot date minus one.
-- **Schema ownership.** The schema is defined by the upstream firehose
-  ingester; [`schema.sql`](schema.sql) is included for reference only.
-  `bsdaily` copies whatever table and index DDL it finds in the source.
-- **Cross-platform deployment.** It is built and run on Linux (the
-  free-space check and fadvise hints use `golang.org/x/sys/unix`; a non-Linux
-  build compiles but is a no-op for the fadvise hints). The hard-coded paths
-  assume the production host's ZFS layout.
+- **Real-time export.** `bsdaily` operates on daily snapshots; the freshest day
+  it can produce is the snapshot date minus one.
+- **Schema ownership.** The schema is defined by the upstream firehose ingester;
+  [`schema.sql`](schema.sql) is included for reference only. `bsdaily` copies
+  whatever table and index DDL it finds in the source.
+- **Cross-platform deployment.** It is built and run on Linux (the free-space
+  check and fadvise hints use `golang.org/x/sys/unix`; a non-Linux build
+  compiles but is a no-op for the fadvise hints). The hard-coded paths assume
+  the production host's ZFS layout.
 
 # How It Works
 
 A single run proceeds as follows:
 
 1. **Find the snapshot.** Scan `SnapshotBase` for directories matching
-   `zfs-auto-snap_daily-YYYY-MM-DD-NNNN`, pick the most recent, and confirm
-   it contains `firehose.db`.
-2. **Determine target days.** Default to the snapshot date minus one day;
-   or use `--date`, or every day in the inclusive `--from`/`--to` range.
+   `zfs-auto-snap_daily-YYYY-MM-DD-NNNN`, pick the most recent, and confirm it
+   contains `firehose.db`.
+2. **Determine target days.** Default to the snapshot date minus one day; or use
+   `--date`, or every day in the inclusive `--from`/`--to` range.
 3. **Preflight disk space.** Require at least 500GiB free on the scratch
    filesystem and 20GiB free on the output filesystem.
 4. **Copy the database to scratch.** Copy `firehose.db`, its `-wal`, and (if
    present) its `-shm` from the snapshot into a fresh temp directory under
    `TmpBase`.
 5. **Per day**, processed strictly one at a time to avoid SQLite contention:
-   - skip the day if its final output file already exists;
-   - `ATTACH` the copied source DB to a new empty per-day DB, recreate the
-     table DDL, and `INSERT ... SELECT` the target day's `posts` plus all
-     rows reachable from them (`posts_hashtags`, `posts_urls`, `hashtags`,
-     `urls`, `users`, and `media` if that table exists);
-   - abort the day cleanly if there are zero posts (`ErrNoPosts`), rather
-     than emitting an empty dump;
-   - recreate indexes, detach the source, and verify the inserted row count;
-   - `sqlite3 .dump | zstdmt` into a hidden temp file;
-   - run a zstd integrity check and confirm the decompressed head looks like
-     SQL;
-   - atomically rename into place and delete the per-day scratch DB.
+    - skip the day if its final output file already exists;
+    - `ATTACH` the copied source DB to a new empty per-day DB, recreate the
+      table DDL, and `INSERT ... SELECT` the target day's `posts` plus all rows
+      reachable from them (`posts_hashtags`, `posts_urls`, `hashtags`, `urls`,
+      `users`, and `media` if that table exists);
+    - abort the day cleanly if there are zero posts (`ErrNoPosts`), rather than
+      emitting an empty dump;
+    - recreate indexes, detach the source, and verify the inserted row count;
+    - `sqlite3 .dump | zstdmt` into a hidden temp file;
+    - run a zstd integrity check and confirm the decompressed head looks like
+      SQL;
+    - atomically rename into place and delete the per-day scratch DB.
 6. **Clean up** the temp directory and log a processed/skipped/total summary.
 
 # Usage
@@ -219,9 +217,9 @@ sqlite3 merged.db < regenerate_auxiliary_tables.sql
 - **Go** (see [`go.mod`](go.mod) for the toolchain version) to build.
 - **Linux** for production use (ZFS snapshots, `statfs` free-space checks,
   `posix_fadvise` hints).
-- The **`sqlite3`** and **`zstdmt`** (multithreaded zstd) binaries on
-  `PATH`; `zstdcat` is used for verification. SQLite reads/writes during
-  extraction use the pure-Go [`modernc.org/sqlite`](https://pkg.go.dev/modernc.org/sqlite)
+- The **`sqlite3`** and **`zstdmt`** (multithreaded zstd) binaries on `PATH`;
+  `zstdcat` is used for verification. SQLite reads/writes during extraction use
+  the pure-Go [`modernc.org/sqlite`](https://pkg.go.dev/modernc.org/sqlite)
   driver, so no cgo is required for that part.
 
 # Configuration
@@ -241,21 +239,21 @@ elsewhere.
 
 # Data Model
 
-The firehose schema (reference copy in [`schema.sql`](schema.sql)) centers
-on a `posts` table, with `users` keyed by DID and many-to-many junction
-tables linking posts to deduplicated `hashtags` and `urls`. An optional
-`media` table tracks downloaded blobs by content hash. `bsdaily` does not
-own this schema; it reflects whatever DDL exists in the source snapshot and
-selects forward from `posts` along the foreign-key relationships to produce a
-referentially-complete per-day slice.
+The firehose schema (reference copy in [`schema.sql`](schema.sql)) centers on a
+`posts` table, with `users` keyed by DID and many-to-many junction tables
+linking posts to deduplicated `hashtags` and `urls`. An optional `media` table
+tracks downloaded blobs by content hash. `bsdaily` does not own this schema; it
+reflects whatever DDL exists in the source snapshot and selects forward from
+`posts` along the foreign-key relationships to produce a referentially-complete
+per-day slice.
 
 # Use Cases
 
 ## Daily public archive
 
-Publish one small, immutable file per day to static HTTP (or IPFS, or a
-mirror network) so that anyone can fetch exactly the day(s) they want and
-re-merge them locally.
+Publish one small, immutable file per day to static HTTP (or IPFS, or a mirror
+network) so that anyone can fetch exactly the day(s) they want and re-merge them
+locally.
 
 ## Backfilling a range
 
@@ -265,16 +263,17 @@ safe to re-run.
 
 ## Reconstituting a full database
 
-Collect any set of daily dumps and merge them with `INSERT OR IGNORE` to
-rebuild a complete, queryable SQLite database, then regenerate the aggregate
-statistics tables.
+Collect any set of daily dumps and merge them with `INSERT OR IGNORE` to rebuild
+a complete, queryable SQLite database, then regenerate the aggregate statistics
+tables.
 
 # See Also
 
 ## Links
 
 - Repo: [https://git.eeqj.de/sneak/bsdaily](https://git.eeqj.de/sneak/bsdaily)
-- Issues: [https://git.eeqj.de/sneak/bsdaily/issues](https://git.eeqj.de/sneak/bsdaily/issues)
+- Issues:
+  [https://git.eeqj.de/sneak/bsdaily/issues](https://git.eeqj.de/sneak/bsdaily/issues)
 - Bluesky: [https://bsky.app](https://bsky.app)
 - zstd: [https://facebook.github.io/zstd/](https://facebook.github.io/zstd/)
 
diff --git a/TODO.md b/TODO.md
index 92ae9e3..91cac96 100644
--- a/TODO.md
+++ b/TODO.md
@@ -1,12 +1,12 @@
 # Workflow
 
-* branch (from `main`)
-* do the work in Next Step
-* move Next Step to the top of Completed Steps
-* move the top item of Future Steps into Next Step
-* commit (`TODO.md` changes in the same commit as the work)
-* merge to `main` if the branch is not protected, otherwise open a PR
-* push
+- branch (from `main`)
+- do the work in Next Step
+- move Next Step to the top of Completed Steps
+- move the top item of Future Steps into Next Step
+- commit (`TODO.md` changes in the same commit as the work)
+- merge to `main` if the branch is not protected, otherwise open a PR
+- push
 
 # Status
 
@@ -14,35 +14,38 @@ pre-1.0
 
 # Next Step
 
-Add the canonical `.golangci.yml`, move the lint phase to golangci-lint
-v2.14.0 in the same commit, and fix the findings it surfaces
+Add the canonical `.golangci.yml`, move the lint phase to golangci-lint v2.14.0
+in the same commit, and fix the findings it surfaces
 (https://git.eeqj.de/sneak/bsdaily/issues/6).
 
 # Completed Steps
 
+- 2026-10-06: Formatted Markdown with prettier: `script/fmt` writes and
+  `script/fmt-check` checks every Markdown file; prettier pinned in
+  `package.json` and `yarn.lock`, installed by `script/bootstrap`, which
+  installs node and yarn at pinned versions when absent and otherwise uses the
+  ones already installed; existing Markdown reformatted.
 - 2026-10-05: Brought the repo up to the standard layout: canonical
-  `.gitignore`, `.dockerignore` and `.editorconfig`; `lint` and `test`
-  phases in the `Dockerfile`, built by `script/lint` and `script/test`;
-  canonical `script/cibuild`, `script/docker` and CI workflow; no linter
-  installed on the host; re-vendored `REPO_POLICIES.md`.
-- 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints,
-  Makefile shims, README Entrypoints section
-- 2026-06-28: Fixed errcheck lint failures; added compilation smoke test;
-  tidied go.mod.
-- 2026-06-28: Added repo scaffolding: README, LICENSE, Makefile,
-  Dockerfile, REPO_POLICIES.md, and Gitea CI.
-- 2026-02-12: Fixed SQLite database locking by removing parallel
-  processing; fixed Linux build via golang.org/x/sys/unix Fadvise.
-- 2026-02-12: Optimized file copy for large databases; moved temp
-  directory to NVMe scratch storage.
+  `.gitignore`, `.dockerignore` and `.editorconfig`; `lint` and `test` phases in
+  the `Dockerfile`, built by `script/lint` and `script/test`; canonical
+  `script/cibuild`, `script/docker` and CI workflow; no linter installed on the
+  host; re-vendored `REPO_POLICIES.md`.
+- 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints, Makefile
+  shims, README Entrypoints section
+- 2026-06-28: Fixed errcheck lint failures; added compilation smoke test; tidied
+  go.mod.
+- 2026-06-28: Added repo scaffolding: README, LICENSE, Makefile, Dockerfile,
+  REPO_POLICIES.md, and Gitea CI.
+- 2026-02-12: Fixed SQLite database locking by removing parallel processing;
+  fixed Linux build via golang.org/x/sys/unix Fadvise.
+- 2026-02-12: Optimized file copy for large databases; moved temp directory to
+  NVMe scratch storage.
 - 2026-02-11: Added date range support.
 - 2026-02-09: Initial implementation: single-day extraction, specific-date
   targeting, faster pruning of throwaway database copies.
 
 # Future Steps
 
-- Format Markdown with prettier in `script/fmt` and `script/fmt-check`
-  (https://git.eeqj.de/sneak/bsdaily/issues/7).
-- Expand tests beyond the compilation smoke test: unit tests for the
-  extraction, verification, and atomic-publish paths.
+- Expand tests beyond the compilation smoke test: unit tests for the extraction,
+  verification, and atomic-publish paths.
 - Cut a first SemVer release once compliance and test coverage land.
diff --git a/package.json b/package.json
new file mode 100644
index 0000000..dc05cde
--- /dev/null
+++ b/package.json
@@ -0,0 +1,5 @@
+{
+  "devDependencies": {
+    "prettier": "3.8.1"
+  }
+}
diff --git a/script/bootstrap b/script/bootstrap
index 4dcdcd7..d2f0739 100755
--- a/script/bootstrap
+++ b/script/bootstrap
@@ -3,11 +3,20 @@
 # this repo. Idempotent: every install is guarded by a check so already
 # installed tools are skipped. Base tooling comes from nix, apt, brew,
 # or apk (detected in that order); assumes NOTHING is present (not git,
-# make, or go).
+# make, go, or node). Node is used directly if installed; otherwise it
+# is installed at a pinned version via nvm (installing nvm itself first,
+# from a hash-verified release archive, never curl | sh).
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
+# Pinned versions, 2026-07-06
+NODE_VERSION="22.17.0"
+NVM_VERSION="0.40.3"
+# sha256 of https://github.com/nvm-sh/nvm/archive/refs/tags/v0.40.3.tar.gz
+NVM_SHA256="5f4d6aaa04a177dc93c985e31dbc411ab6b8c6e1e21d8015dbc1372625fcd1d0"
+YARN_VERSION="1.22.22"
+
 PKGMGR=""
 SUDO=""
 APT_UPDATED=""
@@ -56,6 +65,69 @@ missing() {
     ! command -v "$1" >/dev/null 2>&1
 }
 
+# verify_sha256 <file> <expected-hash>
+verify_sha256() {
+    if command -v sha256sum >/dev/null 2>&1; then
+        actual="$(sha256sum "$1" | cut -d' ' -f1)"
+    else
+        actual="$(shasum -a 256 "$1" | cut -d' ' -f1)"
+    fi
+    if [ "$actual" != "$2" ]; then
+        echo "bootstrap: sha256 mismatch for $1" >&2
+        echo "  expected: $2" >&2
+        echo "  actual:   $actual" >&2
+        exit 1
+    fi
+}
+
+# nvm is a bash script; run a command in a bash with nvm loaded
+nvm_sh() {
+    bash -c ". \"\$HOME/.nvm/nvm.sh\" && $*"
+}
+
+ensure_nvm() {
+    [ -s "$HOME/.nvm/nvm.sh" ] && return 0
+    # nvm prerequisites; nvm itself requires bash
+    if missing bash; then pkg_install bash bash bash bash; fi
+    if missing curl; then pkg_install curl curl curl curl; fi
+    if missing git; then pkg_install git git git git; fi
+    tmp="$(mktemp -d)"
+    curl -fsSL -o "$tmp/nvm.tar.gz" \
+        "https://github.com/nvm-sh/nvm/archive/refs/tags/v${NVM_VERSION}.tar.gz"
+    verify_sha256 "$tmp/nvm.tar.gz" "$NVM_SHA256"
+    mkdir -p "$HOME/.nvm"
+    tar -xzf "$tmp/nvm.tar.gz" -C "$HOME/.nvm" --strip-components=1
+    rm -rf "$tmp"
+}
+
+ensure_node() {
+    if ! missing node; then return 0; fi
+    ensure_nvm
+    nvm_sh "nvm install $NODE_VERSION"
+}
+
+ensure_yarn() {
+    if ! missing yarn; then return 0; fi
+    if ! missing corepack; then
+        corepack enable
+        corepack prepare "yarn@$YARN_VERSION" --activate
+    elif [ -s "$HOME/.nvm/nvm.sh" ]; then
+        nvm_sh "nvm use $NODE_VERSION >/dev/null && corepack enable && \
+            corepack prepare yarn@$YARN_VERSION --activate"
+    else
+        npm install -g "yarn@$YARN_VERSION"
+    fi
+}
+
+install_js_deps() {
+    if missing yarn && [ -s "$HOME/.nvm/nvm.sh" ]; then
+        nvm_sh "nvm use $NODE_VERSION >/dev/null && cd \"$ROOT\" && \
+            yarn install --frozen-lockfile"
+    else
+        yarn install --frozen-lockfile
+    fi
+}
+
 main() {
     cd "$ROOT"
 
@@ -68,6 +140,12 @@ main() {
 
     go mod download
 
+    # Node and yarn, then the prettier pinned in package.json and
+    # yarn.lock, which script/fmt and script/fmt-check run
+    ensure_node
+    ensure_yarn
+    install_js_deps
+
     echo "bootstrap complete"
 }
 
diff --git a/script/fmt b/script/fmt
index e95d111..99e1272 100755
--- a/script/fmt
+++ b/script/fmt
@@ -1,12 +1,34 @@
 #!/bin/sh
-# script/fmt: format all files (writes).
+# script/fmt: format all files (writes): the Go code with go fmt and
+# every Markdown file with prettier.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
+# Must match the pin in script/bootstrap.
+NODE_VERSION="22.17.0"
+
+# script/bootstrap installs node and yarn under nvm and leaves neither
+# on the PATH of the shell that called it, so resolve the pinned
+# toolchain here the way bootstrap's own install step does. nvm is a
+# bash script, hence the subshell.
+run_yarn() {
+    if command -v yarn >/dev/null 2>&1; then
+        exec yarn "$@"
+    fi
+    if [ ! -s "$HOME/.nvm/nvm.sh" ]; then
+        echo "fmt: no yarn; run script/bootstrap first" >&2
+        exit 1
+    fi
+    exec bash -c '. "$HOME/.nvm/nvm.sh" && nvm use "$1" >/dev/null &&
+        shift && exec yarn "$@"' bash "$NODE_VERSION" "$@"
+}
+
 main() {
     cd "$ROOT"
     go fmt ./...
+    # run_yarn replaces this shell, so it stays the last step.
+    run_yarn run prettier --write '**/*.md' --tab-width 4 --prose-wrap always
 }
 
 main "$@"
diff --git a/script/fmt-check b/script/fmt-check
index 57e8e5c..0a6e935 100755
--- a/script/fmt-check
+++ b/script/fmt-check
@@ -1,10 +1,30 @@
 #!/bin/sh
 # script/fmt-check: check formatting (read-only). Same scope as
-# script/fmt, but fails instead of writing.
+# script/fmt, the Go code and every Markdown file, but fails instead of
+# writing.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
+# Must match the pin in script/bootstrap.
+NODE_VERSION="22.17.0"
+
+# script/bootstrap installs node and yarn under nvm and leaves neither
+# on the PATH of the shell that called it, so resolve the pinned
+# toolchain here the way bootstrap's own install step does. nvm is a
+# bash script, hence the subshell.
+run_yarn() {
+    if command -v yarn >/dev/null 2>&1; then
+        exec yarn "$@"
+    fi
+    if [ ! -s "$HOME/.nvm/nvm.sh" ]; then
+        echo "fmt-check: no yarn; run script/bootstrap first" >&2
+        exit 1
+    fi
+    exec bash -c '. "$HOME/.nvm/nvm.sh" && nvm use "$1" >/dev/null &&
+        shift && exec yarn "$@"' bash "$NODE_VERSION" "$@"
+}
+
 main() {
     cd "$ROOT"
     unformatted="$(gofmt -l .)"
@@ -13,6 +33,8 @@ main() {
         echo "$unformatted" >&2
         exit 1
     fi
+    # run_yarn replaces this shell, so it stays the last step.
+    run_yarn run prettier --check '**/*.md' --tab-width 4 --prose-wrap always
 }
 
 main "$@"
diff --git a/yarn.lock b/yarn.lock
new file mode 100644
index 0000000..d846639
--- /dev/null
+++ b/yarn.lock
@@ -0,0 +1,8 @@
+# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.
+# yarn lockfile v1
+
+
+prettier@3.8.1:
+  version "3.8.1"
+  resolved "https://registry.yarnpkg.com/prettier/-/prettier-3.8.1.tgz#edf48977cf991558f4fcbd8a3ba6015ba2a3a173"
+  integrity sha512-UOnG6LftzbdaHZcKoPFtOcCKztrQ57WkHDeRD9t/PTQtmT0NHSeWWepj6pS0z/N7+08BHFDQVUrfmfMRcZwbMg==