Format Markdown with prettier in make fmt (closes #43)

script/fmt and script/fmt-check now run prettier on every Markdown file
after their Go steps, as the standard scripts from sneak/prompts at
dd4027b do. script/bootstrap gains the standard node, nvm and yarn
install next to its goimports install, and the last Dockerfile stage
copies package.json and yarn.lock before running it. .prettierrc,
.prettierignore and yarn.lock are the standard copies. package.json is
the standard one without its licence field and marked private, because
this repo's licence is the WTFPL. .dockerignore keeps the log of a failed
yarn install out of the build context. The existing Markdown is
reformatted in the next commit.

Model: opus-5-5

TODO.md

@@ -22,6 +22,10 @@ Started, Rationale, Design, TODO, License, Author
 
 # Completed Steps
 
+* 2026-10-06: `make fmt` and `make fmt-check` also format Markdown, with
+    `prettier` at the standard settings, pinned through `package.json` and
+    `yarn.lock`; `script/bootstrap` installs node and yarn for it, and the
+    existing Markdown was formatted once
 * 2026-10-06: re-vendored the standard files from `sneak/prompts` at
     commit `dd4027b`: `REPO_POLICIES.md`, the CI workflow, `.gitignore`,
     and `.golangci.yml` together with the lint stage's new image, plus