Format Markdown with prettier in script/fmt and script/fmt-check

script/fmt and script/fmt-check now run prettier over all Markdown
after the Go step. The prettier config, package.json, yarn.lock and the
node and yarn steps of script/bootstrap are copied from sneak/prompts
with the same pinned versions.

script/bootstrap also installs curl when missing: script/install-go
needs it, so a host with only docker and git stopped there.

Model: opus-5-5

README.md

@@ -957,7 +957,9 @@ development workflow, and the Makefile targets are thin shims that call
 them. We provide:
 
 * `script/bootstrap` — install all development dependencies (Go, Go
-  module download). A host without Go gets the `go.mod` version through
+  module download, and node, yarn and `prettier` for Markdown). Node
+  comes from the host, or else at a pinned version through `nvm`. A
+  host without Go gets the `go.mod` version through
   `script/install-go`, in `.tool/go`, which `script/bootstrap` itself,
   the `Makefile`, `script/fmt`, `script/fmt-check`, `script/precommit`
   and `script/release` add to their `PATH`. It
@@ -1011,9 +1013,11 @@ them. We provide:
   needs a *local* daemon. It is a developer convenience and never a
   gate: no gate reads its exit status. Run `make lint` afterwards to
   find out whether the tree is clean.
-* `script/fmt` — format all code (writes)
+* `script/fmt` — format all code (writes): Go with `go fmt`, Markdown
+  with `prettier`
 * `script/fmt-check` — check formatting (read-only). It runs `gofmt` on
-  the host, over every Go file outside `.tool`.
+  the host, over every Go file outside `.tool`, and `prettier --check`
+  over every Markdown file.
 * `script/check` — run `script/test`, `script/lint`, and
   `script/fmt-check`.
 * `script/docker` — build the Docker image tagged via