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/agents/issue_agent.py

@@ -479,6 +479,12 @@ PR reviews run automatically when you open or update a pull request. The bot pro
 - Security vulnerability scanning
 - Approval or change-request recommendations
 
+**Manual re-review:**
+- `{mention_prefix} review-again` - Re-run AI review on current PR state (in PR comments)
+  - Shows diff from previous review (resolved/new issues)
+  - Updates labels and recommendations
+  - Useful after addressing feedback or updating config
+
 ---
 
 ### Quick Examples