feat: Add automatic PR summary generator

Implements automatic PR summary generation feature that analyzes pull
request diffs and generates comprehensive summaries.

Features:
- Auto-generates summaries for PRs with empty descriptions
- Manual trigger via @codebot summarize command in PR comments
- Structured output with change type, files affected, and impact assessment
- Configurable (enable/disable, comment vs description update)

Implementation:
- Added pr_summary.md prompt template for LLM
- Extended PRAgent with summary generation methods
- Added auto_summary configuration in config.yml
- Comprehensive test suite with 10 new tests
- Updated documentation in README.md and CLAUDE.md

Usage:
- Automatic: Opens PR with no description → auto-generates summary
- Manual: Comment '@codebot summarize' on any PR

Related: Issue #2 - Milestone 2 feature delivery

README.md

@@ -9,9 +9,10 @@ Enterprise-grade AI code review system for **Gitea** with automated PR review, i
 | Feature | Description |
 |---------|-------------|
 | **PR Review** | Inline comments, security scanning, severity-based CI failure |
+| **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` in issue comments |
+| **@codebot Commands** | `@codebot summarize`, `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 |
@@ -189,8 +190,51 @@ In any PR comment:
 
 | Command | Description |
 |---------|-------------|
+| `@codebot summarize` | Generate a comprehensive PR summary with changes, files affected, and impact |
 | `@codebot review-again` | Re-run AI code review on current PR state without new commits |
 
+#### PR Summary (`@codebot summarize`)
+
+**Features:**
+- 📋 Generates structured summary of PR changes
+- ✨ Categorizes change type (Feature/Bugfix/Refactor/Documentation/Testing)
+- 📝 Lists what was added, modified, and removed
+- 📁 Shows all affected files with descriptions
+- 🎯 Assesses impact scope (small/medium/large)
+- 🤖 Automatically generates on PRs with empty descriptions
+
+**When to use:**
+- When a PR lacks a description
+- To quickly understand what changed
+- For standardized PR documentation
+- Before reviewing complex PRs
+
+**Example output:**
+```markdown
+## 📋 Pull Request Summary
+This PR implements automatic PR summary generation...
+
+**Type:** ✨ Feature
+
+## Changes
+✅ Added:
+- PR summary generation in PRAgent
+- Auto-summary for empty PR descriptions
+
+📝 Modified:
+- Updated config.yml with new settings
+
+## Files Affected
+- ➕ `tools/ai-review/prompts/pr_summary.md` - New prompt template
+- 📝 `tools/ai-review/agents/pr_agent.py` - Added summary methods
+
+## Impact
+🟡 **Scope:** Medium
+Adds new feature without affecting existing functionality
+```
+
+#### Review Again (`@codebot review-again`)
+
 **Features:**
 - ✅ Shows diff from previous review (resolved/new/changed issues)
 - 🏷️ Updates labels based on new severity