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

CLAUDE.md

@@ -208,6 +208,7 @@ Key workflow pattern:
 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
 - `issue_triage.md` - Issue classification template
 - `issue_response.md` - Issue response template
 
@@ -407,6 +408,59 @@ pytest tests/test_ai_review.py::TestSecurityScanner -v
 
 ## Common Development Tasks
 
+### PR Summary Generation
+
+The PR summary feature automatically generates comprehensive summaries for pull requests.
+
+**Key Features:**
+- Auto-generates summary for PRs with empty descriptions
+- Can be manually triggered with `@codebot summarize` in PR comments
+- Analyzes diff to extract key changes, files affected, and impact
+- Categorizes change type (Feature/Bugfix/Refactor/Documentation/Testing)
+- Posts as comment or updates PR description (configurable)
+
+**Implementation Details:**
+
+1. **Auto-Summary on PR Open** - `PRAgent.execute()`:
+   - Checks if PR body is empty and `auto_summary.enabled` is true
+   - Calls `_generate_pr_summary()` automatically
+   - Continues with normal PR review after posting summary
+
+2. **Manual Trigger** - `@codebot summarize` in PR comments:
+   - `PRAgent.can_handle()` detects `summarize` command in PR comments
+   - Routes to `_handle_summarize_command()`
+   - Generates and posts summary on demand
+
+3. **Summary Generation** - `_generate_pr_summary()`:
+   - Fetches PR diff using `_get_diff()`
+   - Loads `prompts/pr_summary.md` template
+   - Calls LLM with diff to analyze changes
+   - Returns structured JSON with summary data
+   - Formats using `_format_pr_summary()`
+   - Posts as comment or updates description based on config
+
+4. **Configuration** - `config.yml`:
+   ```yaml
+   agents:
+     pr:
+       auto_summary:
+         enabled: true  # Auto-generate for empty PRs
+         post_as_comment: true  # true = comment, false = update description
+   ```
+
+**Summary Structure:**
+- Brief 2-3 sentence overview
+- Change type categorization (Feature/Bugfix/Refactor/etc)
+- Key changes (Added/Modified/Removed)
+- Files affected with descriptions
+- Impact assessment (scope: small/medium/large)
+
+**Common Use Cases:**
+- Developers who forget to write PR descriptions
+- Quick understanding of complex changes
+- Standardized documentation format
+- Pre-review context for reviewers
+
 ### Review-Again Command Implementation
 
 The `@codebot review-again` command allows manual re-triggering of PR reviews without new commits.
@@ -463,6 +517,7 @@ Example commands:
 - `@codebot triage` - Full issue triage with labeling
 - `@codebot explain` - Explain the issue
 - `@codebot suggest` - Suggest solutions
+- `@codebot summarize` - Generate PR summary or issue summary (works on both)
 - `@codebot setup-labels` - Automatic label setup (built-in, not in config)
 - `@codebot review-again` - Re-run PR review without new commits (PR comments only)