chore: prettier-ignore the generated CHANGELOG.md

semantic-release rewrites CHANGELOG.md on every release in its own
style, which left format:check failing on main after each release
commit. Treat it like routeTree.gen.ts: generated, never
hand-formatted. Also ignore .vercel/.nitro build output.

Author: claude

.prettierignore

@@ -2,7 +2,11 @@ node_modules
 dist
 .output
 .vinxi
+.vercel
+.nitro
 pnpm-lock.yaml
 package-lock.json
 bun.lock
 routeTree.gen.ts
+# Generated by semantic-release on every release; never hand-formatted.
+CHANGELOG.md