Retire unused changelog

[pglevy]

1. Replace CHANGELOG.md with a short pointer, not a full history:

     # Changelog

     Release notes are maintained in GitHub Releases:
     https://github.com/pglevy/sailwind/releases

     Breaking changes and migration notes are documented in each release.

2. Update PUBLISHING.md / release checklist to require:
   • Breaking Changes
   • Migration Notes
   • Added / Changed / Fixed
   • linked PRs/issues