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

README.md

@@ -169,6 +169,8 @@ python main.py chat owner/repo "Find all API endpoints" --issue 789
 
 ## @codebot Commands
 
+### Issue Commands
+
 In any issue comment:
 
 | Command | Description |
@@ -181,6 +183,32 @@ In any issue comment:
 | `@codebot suggest` | Suggest solutions or next steps |
 | `@codebot` (any question) | Chat with AI using codebase/web search tools |
 
+### Pull Request Commands
+
+In any PR comment:
+
+| Command | Description |
+|---------|-------------|
+| `@codebot review-again` | Re-run AI code review on current PR state without new commits |
+
+**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
+
+**When to use:**
+- After addressing review feedback in comments
+- When AI flagged a false positive and you explained it
+- After updating `.ai-review.yml` security rules
+- To re-evaluate severity after code clarification
+
+**Example:**
+```
+The hardcoded string at line 45 is a public API URL, not a secret.
+@codebot review-again
+```
+
 **New to OpenRabbit?** Just type `@codebot help` in any issue to see all available commands!
 
 ### Label Setup Command