GitLab

- Backfill CHANGELOG.md with v2.4.0 entry summarizing the
  v2.1.112 → v2.1.119 sync work
- Refresh SECURITY.md footer (Last Updated April 27, 2026;
  Next Review April 2027)
- Fix model ID in 09-advanced-features/config-examples.json:
  claude-opus-4 → claude-opus-4-7 (3 occurrences) for
  consistency with documented model IDs
- Remove RELEASE_NOTES.md stub that duplicated and contradicted
  CHANGELOG.md