docs: sync to Claude Code v2.1.119 (delta from 2026-04-24 plan) (#102)

- 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

SECURITY.md

@@ -326,8 +326,8 @@ This security policy is reviewed and updated:
 - When the project scope changes
 - Annually as a minimum
 
-**Last Updated**: January 2026
-**Next Review**: January 2027
+**Last Updated**: April 27, 2026
+**Next Review**: April 2027
 
 ---