Format Markdown with prettier in make fmt and fmt-check (closes #23)

`make fmt` now runs prettier over the Markdown files after the Go
formatters, and `make fmt-check` fails when prettier would change one.
`package.json`, `yarn.lock`, `.prettierrc` and `.prettierignore` are the
`sneak/prompts` copies at `dd4027b`, except for `"private": true` in
`package.json` in place of the licence field. Both scripts find yarn the
way that repo's scripts do. `script/bootstrap` installs prettier with
`yarn install --frozen-lockfile`. When the yarn those scripts would run
is not 1.22.22, it installs yarn through corepack under node 22.17.0:
the node on `PATH` if it has that version, otherwise one installed
through nvm from a hash-checked archive. `README.md` and `TODO.md` are
reformatted once.

Model: opus-5-5

TODO.md

@@ -1,85 +1,83 @@
 # 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
 
 1.0+
 
-Tagged 1.0.0 (2025-05-08). Substantial correctness fixes and features
-have landed since the tag.
+Tagged 1.0.0 (2025-05-08). Substantial correctness fixes and features have
+landed since the tag.
 
 # Next Step
 
-Restructure README.md into the standard sections: Description, Getting
-Started, Entrypoints, Rationale, Design, TODO, License, Author (Getting
-Started, Why?, TODO, License exist; Description, Entrypoints, Design,
-Author are missing)
+Restructure README.md into the standard sections: Description, Getting Started,
+Entrypoints, Rationale, Design, TODO, License, Author (Getting Started, Why?,
+TODO, License exist; Description, Entrypoints, Design, Author are missing)
 
 # Completed Steps
 
-* 2026-10-06: `script/fmt-check` checks what `script/fmt` writes: it
-    fails and lists the files when `gofmt -s` or goimports would change
-    one, so a file that passes `make check` no longer changes on the
-    next `make fmt`
-* 2026-10-06: canonical files re-vendored from `sneak/prompts` at
-    `dd4027b`: `REPO_POLICIES.md` and `.editorconfig` added;
-    `.dockerignore`, `.gitignore`, `.golangci.yml` and the workflow
-    refreshed, keeping this repo's own entries and `fetch-depth: 0`;
-    the lint phase runs golangci-lint v2.14.0; `script/bootstrap`
-    installs goimports unless the installed one has the pinned version,
-    and it and `script/fmt` put Go's bin directory on `PATH`
-* 2026-10-06: the summary line after `sum`, `check` and `clear` prints
-    the byte unit once (`1.5 KiB`, not `1.5 KiB bytes`)
-* 2026-10-06: a path that `--exclude` or `--exclude-dotfiles` excludes
-    is skipped even when it cannot be read, so an excluded directory
-    that cannot be listed no longer fails the run
-* 2026-10-06: lint and test run as phases of the `Dockerfile`, and the
-    build stage depends on both; `script/lint` and `script/test` each
-    build their phase with `--no-cache`; `script/cibuild` bootstraps,
-    runs `script/check`, then builds the image; golangci-lint is no
-    longer installed on the host
-* 2026-10-06: `check --continue` keeps going past a file or directory
-    it cannot read: it counts it as failed, prints the error and the
-    path on stderr, and checks the rest of the tree
-* 2026-10-05: golangci-lint settings take effect: canonical
-    `.golangci.yml` (v2 layout, settings under `linters.settings`),
-    golangci-lint pinned at v2.12.2 in `Dockerfile` and
-    `script/bootstrap`, and the code fixed for what the settings now
-    report (long lines in `attrsum.go` rewrapped)
-* 2026-10-05: `make try` runs on three small files in a temporary
-    directory that it removes afterwards, also when a step fails,
-    instead of on a fixed directory on one person's machine
-* 2026-10-02: `attrsum --version` reports the git tag or short commit,
-    stamped by `make build` and by a plain `docker build .` of a clone;
-    `.dockerignore` sends `.git` without `.git/config` and keeps a
-    host-built `attrsum` out; CI checks out full history so it stamps
-    the same version
-* 2026-02-02: correctness pass: track actual bytes read instead of
-    stale file size, atomic failure tracking in ProcessCheck, detect
-    file modification during checksum (TOCTOU), propagate countFiles
-    errors, single progress bar across paths, error on empty stdin,
-    dead code removal
-* 2026-02-01: added quiet mode, progress bar, summary report, and stdin
-    path input; multiple file/directory arguments for all commands
-* 2025-07-12: README update
-* 2025-05-08: initial working tool with passing tests, skips
-    non-regular files, Makefile, README; tagged 1.0.0
+- 2026-10-06: `make fmt` formats the Markdown files with prettier (four-space
+  indents, `proseWrap: always`) and `make fmt-check` fails on one it would
+  change; prettier is pinned in `package.json` and `yarn.lock`, and
+  `script/bootstrap` installs it, along with the pinned node and yarn when the
+  yarn on hand is not the pinned version; the Markdown files reformatted once
+- 2026-10-06: `script/fmt-check` checks what `script/fmt` writes: it fails and
+  lists the files when `gofmt -s` or goimports would change one, so a file that
+  passes `make check` no longer changes on the next `make fmt`
+- 2026-10-06: canonical files re-vendored from `sneak/prompts` at `dd4027b`:
+  `REPO_POLICIES.md` and `.editorconfig` added; `.dockerignore`, `.gitignore`,
+  `.golangci.yml` and the workflow refreshed, keeping this repo's own entries
+  and `fetch-depth: 0`; the lint phase runs golangci-lint v2.14.0;
+  `script/bootstrap` installs goimports unless the installed one has the pinned
+  version, and it and `script/fmt` put Go's bin directory on `PATH`
+- 2026-10-06: the summary line after `sum`, `check` and `clear` prints the byte
+  unit once (`1.5 KiB`, not `1.5 KiB bytes`)
+- 2026-10-06: a path that `--exclude` or `--exclude-dotfiles` excludes is
+  skipped even when it cannot be read, so an excluded directory that cannot be
+  listed no longer fails the run
+- 2026-10-06: lint and test run as phases of the `Dockerfile`, and the build
+  stage depends on both; `script/lint` and `script/test` each build their phase
+  with `--no-cache`; `script/cibuild` bootstraps, runs `script/check`, then
+  builds the image; golangci-lint is no longer installed on the host
+- 2026-10-06: `check --continue` keeps going past a file or directory it cannot
+  read: it counts it as failed, prints the error and the path on stderr, and
+  checks the rest of the tree
+- 2026-10-05: golangci-lint settings take effect: canonical `.golangci.yml` (v2
+  layout, settings under `linters.settings`), golangci-lint pinned at v2.12.2 in
+  `Dockerfile` and `script/bootstrap`, and the code fixed for what the settings
+  now report (long lines in `attrsum.go` rewrapped)
+- 2026-10-05: `make try` runs on three small files in a temporary directory that
+  it removes afterwards, also when a step fails, instead of on a fixed directory
+  on one person's machine
+- 2026-10-02: `attrsum --version` reports the git tag or short commit, stamped
+  by `make build` and by a plain `docker build .` of a clone; `.dockerignore`
+  sends `.git` without `.git/config` and keeps a host-built `attrsum` out; CI
+  checks out full history so it stamps the same version
+- 2026-02-02: correctness pass: track actual bytes read instead of stale file
+  size, atomic failure tracking in ProcessCheck, detect file modification during
+  checksum (TOCTOU), propagate countFiles errors, single progress bar across
+  paths, error on empty stdin, dead code removal
+- 2026-02-01: added quiet mode, progress bar, summary report, and stdin path
+  input; multiple file/directory arguments for all commands
+- 2025-07-12: README update
+- 2025-05-08: initial working tool with passing tests, skips non-regular files,
+  Makefile, README; tagged 1.0.0
 
 # Future Steps
 
-* Add a `LICENSE` file matching the README's WTFPL v2; sneak's to add,
-    not an agent's
-* Tag a patch release to ship the 2026-02-02 correctness fixes
-* Dry-run mode (--dry-run, -n): show what would be done without making
-    changes (from README TODO)
-* JSON output (--json) for scripting and integration (from README TODO)
-* Parallel processing (-j N) with multiple goroutines for faster
-    checksumming on large trees (from README TODO)
-* Formalize and document exit codes for scripting (from README TODO)
+- Add a `LICENSE` file matching the README's WTFPL v2; sneak's to add, not an
+  agent's
+- Tag a patch release to ship the 2026-02-02 correctness fixes
+- Dry-run mode (--dry-run, -n): show what would be done without making changes
+  (from README TODO)
+- JSON output (--json) for scripting and integration (from README TODO)
+- Parallel processing (-j N) with multiple goroutines for faster checksumming on
+  large trees (from README TODO)
+- Formalize and document exit codes for scripting (from README TODO)