Format the markdown with prettier in script/fmt and script/fmt-check (closes #100)

script/fmt and script/fmt-check run prettier 3.8.1 on the markdown after
gofmt, with the same yarn helper and arguments as the copies in
sneak/prompts. prettier is pinned in package.json and yarn.lock;
.prettierrc sets four-space tabs and proseWrap always, and
.prettierignore keeps prettier off REPO_POLICIES.md and vendor/. Plain
script/bootstrap now installs Node and Yarn the way the one in
sneak/prompts does, then prettier; with --cgo it does not, as the
Dockerfile stages that pass it format nothing. The HTML templates stay
out: prettier cannot parse a Go template action inside a tag.

Model: opus-5-5

TODO.md

@@ -31,6 +31,16 @@ P2: security: per-IP rate limiting on the image routes
 
 # Completed Steps
 
+- 2026-10-05 the markdown is formatted with prettier (closes #100):
+  `script/fmt` and `script/fmt-check` run prettier 3.8.1, pinned in
+  `package.json` and `yarn.lock`, on `**/*.md` after `gofmt`, with four-space
+  tabs and `proseWrap: always` as `.prettierrc` says; `.prettierignore` keeps
+  it off `REPO_POLICIES.md`, the copy from `sneak/prompts`, and `vendor/`.
+  Plain `script/bootstrap` installs Node and Yarn as the one in `sneak/prompts`
+  does and then prettier; `script/bootstrap --cgo` does not, as the
+  `Dockerfile` stages that run it format nothing. The HTML templates stay
+  unformatted: prettier cannot parse a Go template action inside a tag. The
+  markdown was reflowed in a commit of its own.
 - 2026-10-05 lint and tests run as the `lint` and `test` phases of the
   `Dockerfile`, built with `--no-cache` (closes #202): `script/check`,
   `script/cibuild`, `script/docker`, `script/lint`, `script/test`,