feat: Add @codebot review-again command for manual PR re-review

- Add review-again command to trigger PR review without new commits
- Implement review comparison logic to show resolved/new/changed issues
- Update workflow to handle PR comments via dispatcher
- Add comprehensive help documentation in README and CLAUDE.md
- Show diff from previous review with resolved/new issues count
- Update PR labels based on new severity assessment
- Support re-evaluation after config changes or false positive clarification

Key features:
- ✅ Shows diff from previous review (resolved/new/changed issues)
- 🏷️ Updates labels based on new severity
- ⚡ No need for empty commits to trigger review
- 🔧 Respects latest .ai-review.yml configuration

Closes feature request for manual PR re-review capability

tools/ai-review/config.yml

@@ -65,6 +65,7 @@ interaction:
         - security
         - summarize
         - triage
+        - review-again
 
 # Enterprise settings
 enterprise: