ci: ignore release-please-generated CHANGELOG.md in prettier check

The CHANGELOG.md file is auto-generated by release-please and doesn't pass
through prettier, causing format:check to fail in CI on every release commit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ktamas77

.prettierignore

@@ -2,3 +2,4 @@ node_modules
 dist
 package-lock.json
 *.tsbuildinfo
+CHANGELOG.md