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

CLAUDE.md

@@ -145,6 +145,7 @@ interaction:
     - security     # Security analysis
     - summarize    # Summarize the issue
     - triage       # Full triage with labeling
+    - review-again # Re-run PR review (PR comments only)
   
 review:
   fail_on_severity: HIGH  # Fail CI if HIGH severity issues found
@@ -309,6 +310,49 @@ pytest tests/test_ai_review.py::TestSecurityScanner -v
 
 ## Common Development Tasks
 
+### Review-Again Command Implementation
+
+The `@codebot review-again` command allows manual re-triggering of PR reviews without new commits.
+
+**Key Features:**
+- Detects `@codebot review-again` in PR comments (not issue comments)
+- Compares new review with previous review to show resolved/new issues
+- Updates existing AI review comment instead of creating duplicates
+- Updates PR labels based on new severity assessment
+
+**Implementation Details:**
+
+1. **PRAgent.can_handle()** - Handles `issue_comment` events on PRs containing "review-again"
+2. **PRAgent._handle_review_again()** - Main handler that:
+   - Fetches previous review comment
+   - Re-runs full PR review (security scan + AI analysis)
+   - Compares findings using `_compare_reviews()`
+   - Generates diff report with `_format_review_update()`
+   - Updates comment and labels
+
+3. **Review Comparison** - Uses finding keys (file:line:description) to match issues:
+   - **Resolved**: Issues in previous but not in current review
+   - **New**: Issues in current but not in previous review
+   - **Still Present**: Issues in both reviews
+   - **Severity Changed**: Same issue with different severity
+
+4. **Workflow Integration** - `.gitea/workflows/ai-comment-reply.yml`:
+   - Detects if comment is on PR or issue
+   - Uses `dispatch` command for PRs to route to PRAgent
+   - Preserves backward compatibility with issue commands
+
+**Usage:**
+```bash
+# In a PR comment:
+@codebot review-again
+```
+
+**Common Use Cases:**
+- Re-evaluate after explaining false positives in comments
+- Test new `.ai-review.yml` configuration
+- Update severity after code clarification
+- Faster iteration without empty commits
+
 ### Adding a New Command to @codebot
 
 1. Add command to `config.yml` under `interaction.commands`
@@ -323,6 +367,7 @@ Example commands:
 - `@codebot explain` - Explain the issue
 - `@codebot suggest` - Suggest solutions
 - `@codebot setup-labels` - Automatic label setup (built-in, not in config)
+- `@codebot review-again` - Re-run PR review without new commits (PR comments only)
 
 ### Changing the Bot Name