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

Replace the unpinned `npx prettier --tab-width 4` in `script/fmt` with the
canonical toolchain vendored from `sneak/prompts`: `.prettierrc` (tabWidth 4,
proseWrap always), pinned `package.json` + `yarn.lock` (prettier 3.8.1), and a
`.prettierignore` that unions the scaffold entries with the repo's `*.min.js`
line. `script/bootstrap` now installs a pinned node/yarn via a hash-verified
nvm release archive (never curl-pipe-sh), so prettier stops being an unpinned
host tool. `script/fmt` runs the pinned prettier via `.prettierrc` over
`static/js/*.js` and `**/*.md`, keeping gofmt/goimports. All existing markdown
is reflowed to house style; `static/js/alpine.min.js` stays byte-identical.

Model: opus-4-8

.prettierrc

@@ -0,0 +1,4 @@
+{
+    "tabWidth": 4,
+    "proseWrap": "always"
+}