Test changelog generation logic in publish workflow

[bntvllnt]

Problem

publish.yml contains inline shell logic for changelog generation. This logic is untested — if it breaks, publish workflow still succeeds but with empty/malformed release notes.

Acceptance Criteria

• Extract changelog generation to a testable script
• Add test validating output format
• Or simplify by using gh release create --generate-notes

Priority

Low — nice-to-have for release quality.