Cleanup changelog

GOAL: fix formatting and remove empty sections in CHANGELOG.md.

WHEN: after generating or updating the changelog.

Efficiency directives

• Optimize all operations for token and context efficiency
• Batch operations on file groups, avoid individual file processing
• Target only relevant files
• Reduce token usage

Workflow

• Invoke the fix-markdown skill on CHANGELOG.md
• Run scripts/remove-empty-headers.sh CHANGELOG.md
• Invoke the fix-markdown skill on CHANGELOG.md again
• DONE