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

CLAUDE.md

@@ -216,6 +216,7 @@ Prompts are stored in `tools/ai-review/prompts/` as Markdown files:
 
 - `base.md` - Base instructions for all reviews
 - `pr_summary.md` - PR summary generation template
+- `changelog.md` - Keep a Changelog format generation template
 - `issue_triage.md` - Issue classification template
 - `issue_response.md` - Issue response template
 
@@ -468,6 +469,70 @@ The PR summary feature automatically generates comprehensive summaries for pull
 - Standardized documentation format
 - Pre-review context for reviewers
 
+### PR Changelog Generation
+
+The `@codebot changelog` command generates Keep a Changelog format entries from PR diffs.
+
+**Key Features:**
+- Generates structured changelog entries following Keep a Changelog format
+- Categorizes changes: Added/Changed/Deprecated/Removed/Fixed/Security
+- Automatically detects breaking changes
+- Includes technical details (files changed, LOC, components)
+- Output is ready to copy-paste into CHANGELOG.md
+
+**Implementation Details:**
+
+1. **Command Handler** - `PRAgent._handle_changelog_command()`:
+   - Triggered by `@codebot changelog` in PR comments
+   - Fetches PR title, description, and diff
+   - Loads `prompts/changelog.md` template
+   - Formats prompt with PR context
+
+2. **LLM Analysis** - Generates structured JSON:
+   ```json
+   {
+     "changelog": {
+       "added": ["New features"],
+       "changed": ["Changes to existing functionality"],
+       "fixed": ["Bug fixes"],
+       "security": ["Security fixes"]
+     },
+     "breaking_changes": ["Breaking changes"],
+     "technical_details": {
+       "files_changed": 15,
+       "insertions": 450,
+       "deletions": 120,
+       "main_components": ["auth/", "api/"]
+     }
+   }
+   ```
+
+3. **Formatting** - `_format_changelog()`:
+   - Converts JSON to Keep a Changelog markdown format
+   - Uses emojis for visual categorization (✨ Added, 🔄 Changed, 🐛 Fixed)
+   - Highlights breaking changes prominently
+   - Includes technical summary at the end
+   - Omits empty sections for clean output
+
+4. **Prompt Engineering** - `prompts/changelog.md`:
+   - User-focused language (not developer jargon)
+   - Filters noise (formatting, typos, minor refactoring)
+   - Groups related changes
+   - Active voice, concise entries
+   - Maximum 100 characters per entry
+
+**Common Use Cases:**
+- Preparing release notes
+- Maintaining CHANGELOG.md
+- Customer-facing announcements
+- Version documentation
+
+**Workflow Safety:**
+- Only triggers on PR comments (not issue comments)
+- Included in ai-comment-reply.yml workflow conditions
+- Excluded from ai-chat.yml to prevent duplicate runs
+- No automatic triggering - manual command only
+
 ### Review-Again Command Implementation
 
 The `@codebot review-again` command allows manual re-triggering of PR reviews without new commits.
@@ -525,6 +590,7 @@ Example commands:
 - `@codebot explain` - Explain the issue
 - `@codebot suggest` - Suggest solutions
 - `@codebot summarize` - Generate PR summary or issue summary (works on both)
+- `@codebot changelog` - Generate Keep a Changelog format entries (PR comments only)
 - `@codebot setup-labels` - Automatic label setup (built-in, not in config)
 - `@codebot review-again` - Re-run PR review without new commits (PR comments only)