Improve release-plan validation PR feedback (comments and file annotations)

[hdamker]

Problem description

The release-plan.yaml validation currently surfaces errors/warnings through:

• Commit status with description
• Job summary in Actions tab
• Annotations in workflow logs

However, finding these results requires navigating to the Actions tab. Users may miss important validation feedback.

Possible evolution

1. PR Comment with validation results

   • Post a comment summarizing validation results (similar to MegaLinter)
   • Show errors/warnings in a formatted table or list
   • Include link to full workflow run for details

2. Comment update logic

   • Identify existing validation comment in subsequent runs
   • Replace with latest status instead of creating new comment
   • Use a marker/identifier in the comment body (like MegaLinter does)

3. File tab annotations

   • Add inline annotations on release-plan.yaml in the Files tab
   • Show errors/warnings at specific lines where possible
   • Similar to how MegaLinter shows yamllint errors inline

Alternative solution

Could use a GitHub App for richer PR integration, but GitHub Actions with github-script should be sufficient for MVP.

Additional context

• Related PR: Add release-plan validation support #48 (Initial release-plan validation)
• Related issue: Validation: Complete release-plan.yaml validation infrastructure #49 (Release plan validation tracker)
• Reference: MegaLinter's PR comment implementation