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

.gitea/workflows/ai-comment-reply.yml

@@ -1,7 +1,7 @@
 name: AI Comment Reply
 
 # WORKFLOW ROUTING:
-# This workflow handles SPECIFIC commands: help, explain, suggest, security, summarize, review-again, setup-labels
+# This workflow handles SPECIFIC commands: help, explain, suggest, security, summarize, changelog, review-again, setup-labels
 # Other workflows: ai-issue-triage.yml (@codebot triage), ai-chat.yml (free-form questions)
 
 on:
@@ -23,6 +23,7 @@ jobs:
              contains(github.event.comment.body, '@codebot suggest') ||
              contains(github.event.comment.body, '@codebot security') ||
              contains(github.event.comment.body, '@codebot summarize') ||
+             contains(github.event.comment.body, '@codebot changelog') ||
              contains(github.event.comment.body, '@codebot review-again') ||
              contains(github.event.comment.body, '@codebot setup-labels'))
         steps: