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

tools/ai-review/config.yml

@@ -32,6 +32,9 @@ agents:
         events:
             - opened
             - synchronize
+        auto_summary:
+            enabled: true # Auto-generate summary for PRs with empty descriptions
+            post_as_comment: true # true = post as comment, false = update PR description
     codebase:
         enabled: true
         schedule: "0 0 * * 0" # Weekly on Sunday
@@ -63,7 +66,7 @@ interaction:
         - explain
         - suggest
         - security
-        - summarize
+        - summarize # Generate PR summary (works on both issues and PRs)
         - triage
         - review-again