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)

package.json

@@ -0,0 +1,5 @@
+{
+  "devDependencies": {
+    "prettier": "3.8.1"
+  }
+}