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

script/fmt-check

@@ -5,6 +5,26 @@ 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
+        yarn "$@"
+        return
+    fi
+    if [ ! -s "$HOME/.nvm/nvm.sh" ]; then
+        echo "fmt-check: no yarn; run script/bootstrap first" >&2
+        exit 1
+    fi
+    bash -c '. "$HOME/.nvm/nvm.sh" && nvm use "$1" >/dev/null &&
+        shift && exec yarn "$@"' bash "$NODE_VERSION" "$@"
+}
+
 main() {
     cd "$ROOT"
     # script/bootstrap installs goimports into Go's bin directory, which
@@ -18,6 +38,7 @@ main() {
         echo "$files" | sort -u >&2
         exit 1
     fi
+    run_yarn run prettier --check '**/*.md' --tab-width 4 --prose-wrap always
 }
 
 main "$@"