Vendor pinned prettier/format toolchain from prompts scaffold (closes #203)

script/fmt ran unpinned npx prettier with an inline --tab-width and no config, so formatting was not reproducible. This vendors the canonical format toolchain from the prompts scaffold: .prettierrc (tabWidth 4, proseWrap always), package.json and yarn.lock pinning prettier 3.8.1, and a script/bootstrap that installs node/yarn from a hash-verified nvm archive. script/fmt now reads the config over static/js and markdown; gofmt/goimports unchanged. .prettierignore keeps the *.min.js rule protecting the vendored alpine.min.js. Existing markdown was reflowed to house style.

Disclosure: make check does not gate prettier; the linter's pre-existing gomodguard deprecation warning is unrelated and left as-is.

Model: opus-4-8 (implementation and review)

script/fmt

@@ -4,11 +4,34 @@ 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: 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"
     gofmt -s -w .
     goimports -w .
-    npx prettier --write --tab-width 4 static/js/*.js
+    # Pinned prettier reads settings from .prettierrc (tabWidth 4,
+    # proseWrap always); .prettierignore keeps alpine.min.js untouched.
+    # Globs are quoted so prettier expands them, not the shell.
+    run_yarn run prettier --write 'static/js/*.js' '**/*.md'
 }
 
 main "$@"