feat: Add @codebot changelog command for Keep a Changelog format generation

Implements PR changelog generator that analyzes diffs and generates
Keep a Changelog format entries ready for CHANGELOG.md.

Features:
- Generates structured changelog entries (Added/Changed/Fixed/etc.)
- Automatically detects breaking changes
- Includes technical details (files, LOC, components)
- User-focused language filtering out noise
- Ready to copy-paste into CHANGELOG.md

Implementation:
- Added changelog.md prompt template with Keep a Changelog format
- Implemented _handle_changelog_command() in PRAgent
- Added _format_changelog() for markdown formatting
- Updated PRAgent.can_handle() to route changelog commands
- Added 'changelog' to config.yml commands list

Workflow Safety (prevents duplicate runs):
- Added '@codebot changelog' to ai-comment-reply.yml conditions
- Excluded from ai-chat.yml to prevent duplication
- Only triggers on PR comments (not issues)
- Manual command only (no automatic triggering)

Testing:
- 9 comprehensive tests in TestChangelogGeneration class
- Tests command detection, formatting, config validation
- Verifies prompt formatting and Keep a Changelog structure

Documentation:
- Updated README.md with changelog command and examples
- Added detailed implementation guide in CLAUDE.md
- Included example output and use cases

Related: Milestone 2 feature - PR changelog generation for release notes

README.md

@@ -12,7 +12,7 @@ Enterprise-grade AI code review system for **Gitea** with automated PR review, i
 | **PR Summaries** | Auto-generate comprehensive PR summaries with change analysis and impact assessment |
 | **Issue Triage** | On-demand classification, labeling, priority assignment via `@codebot triage` |
 | **Chat** | Interactive AI chat with codebase search and web search tools |
-| **@codebot Commands** | `@codebot summarize`, `explain`, `suggest`, `triage`, `review-again` in comments |
+| **@codebot Commands** | `@codebot summarize`, `changelog`, `explain`, `suggest`, `triage`, `review-again` in comments |
 | **Codebase Analysis** | Health scores, tech debt tracking, weekly reports |
 | **Security Scanner** | 17 OWASP-aligned rules for vulnerability detection |
 | **Enterprise Ready** | Audit logging, metrics, Prometheus export |
@@ -191,6 +191,7 @@ In any PR comment:
 | Command | Description |
 |---------|-------------|
 | `@codebot summarize` | Generate a comprehensive PR summary with changes, files affected, and impact |
+| `@codebot changelog` | Generate Keep a Changelog format entries ready for CHANGELOG.md |
 | `@codebot review-again` | Re-run AI code review on current PR state without new commits |
 
 #### PR Summary (`@codebot summarize`)
@@ -233,6 +234,53 @@ This PR implements automatic PR summary generation...
 Adds new feature without affecting existing functionality
 ```
 
+#### Changelog Generator (`@codebot changelog`)
+
+**Features:**
+- 📋 Generates Keep a Changelog format entries
+- 🏷️ Categorizes changes (Added/Changed/Fixed/Removed/Security)
+- ⚠️ Detects breaking changes automatically
+- 📊 Includes technical details (files, LOC, components)
+- 📝 Ready to copy-paste into CHANGELOG.md
+
+**When to use:**
+- Preparing release notes
+- Maintaining CHANGELOG.md
+- Customer-facing announcements
+- Version documentation
+
+**Example output:**
+```markdown
+## 📋 Changelog for PR #123
+
+### ✨ Added
+- User authentication system with JWT tokens
+- Password reset functionality via email
+
+### 🔄 Changed
+- Updated database schema for user table
+- Refactored login endpoint for better error handling
+
+### 🐛 Fixed
+- Session timeout bug causing premature logouts
+- Security vulnerability in password validation
+
+### 🔒 Security
+- Fixed XSS vulnerability in user input validation
+
+---
+
+### ⚠️ BREAKING CHANGES
+- **Removed legacy API endpoint /api/v1/old - migrate to /api/v2**
+
+---
+
+### 📊 Technical Details
+- **Files changed:** 15
+- **Lines:** +450 / -120
+- **Main components:** auth/, api/users/, database/
+```
+
 #### Review Again (`@codebot review-again`)
 
 **Features:**