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

tests/test_ai_review.py

@@ -588,5 +588,242 @@ class TestLabelSetup:
         assert "Kind/Bug" in config["aliases"]
 
 
+class TestPRSummaryGeneration:
+    """Test PR summary generation functionality."""
+
+    def test_pr_summary_prompt_exists(self):
+        """Verify pr_summary.md prompt file exists."""
+        prompt_path = os.path.join(
+            os.path.dirname(__file__),
+            "..",
+            "tools",
+            "ai-review",
+            "prompts",
+            "pr_summary.md",
+        )
+        assert os.path.exists(prompt_path), "pr_summary.md prompt file not found"
+
+    def test_pr_summary_prompt_formatting(self):
+        """Test that pr_summary.md can be loaded without errors."""
+        prompt_path = os.path.join(
+            os.path.dirname(__file__),
+            "..",
+            "tools",
+            "ai-review",
+            "prompts",
+            "pr_summary.md",
+        )
+        with open(prompt_path) as f:
+            prompt = f.read()
+
+        # Check for key elements
+        assert "summary" in prompt.lower()
+        assert "change_type" in prompt.lower()
+        assert "files_affected" in prompt.lower()
+        assert "impact" in prompt.lower()
+        assert "JSON" in prompt
+
+        # Verify JSON examples use double curly braces (escaped)
+        # Should not raise KeyError when formatted with empty string
+        try:
+            formatted = prompt.format()
+        except KeyError as e:
+            pytest.fail(f"Prompt has unescaped placeholders: {e}")
+
+    def test_pr_agent_has_summary_marker(self):
+        """Verify PRAgent has PR_SUMMARY_MARKER constant."""
+        from agents.pr_agent import PRAgent
+
+        assert hasattr(PRAgent, "PR_SUMMARY_MARKER")
+        assert PRAgent.PR_SUMMARY_MARKER == "<!-- AI_PR_SUMMARY -->"
+
+    def test_pr_agent_can_handle_summarize_command(self):
+        """Test that PRAgent can handle @codebot summarize in PR comments."""
+        from agents.pr_agent import PRAgent
+
+        config = {
+            "agents": {
+                "pr": {
+                    "enabled": True,
+                }
+            },
+            "interaction": {
+                "mention_prefix": "@codebot",
+            },
+        }
+
+        agent = PRAgent(config=config)
+
+        # Test summarize command in PR comment
+        event_data = {
+            "action": "created",
+            "issue": {
+                "number": 123,
+                "pull_request": {},  # Indicates this is a PR
+            },
+            "comment": {
+                "body": "@codebot summarize this PR please",
+            },
+        }
+
+        assert agent.can_handle("issue_comment", event_data) is True
+
+    def test_pr_agent_can_handle_summarize_case_insensitive(self):
+        """Test that summarize command is case-insensitive."""
+        from agents.pr_agent import PRAgent
+
+        config = {
+            "agents": {
+                "pr": {
+                    "enabled": True,
+                }
+            },
+            "interaction": {
+                "mention_prefix": "@codebot",
+            },
+        }
+
+        agent = PRAgent(config=config)
+
+        # Test various casings
+        for body in [
+            "@codebot SUMMARIZE",
+            "@codebot Summarize",
+            "@codebot SuMmArIzE",
+        ]:
+            event_data = {
+                "action": "created",
+                "issue": {
+                    "number": 123,
+                    "pull_request": {},
+                },
+                "comment": {"body": body},
+            }
+            assert agent.can_handle("issue_comment", event_data) is True
+
+    def test_pr_agent_ignores_summarize_on_non_pr(self):
+        """Test that summarize command is ignored on regular issues."""
+        from agents.pr_agent import PRAgent
+
+        config = {
+            "agents": {
+                "pr": {
+                    "enabled": True,
+                }
+            },
+            "interaction": {
+                "mention_prefix": "@codebot",
+            },
+        }
+
+        agent = PRAgent(config=config)
+
+        # Regular issue (no pull_request field)
+        event_data = {
+            "action": "created",
+            "issue": {
+                "number": 123,
+                # No pull_request field
+            },
+            "comment": {
+                "body": "@codebot summarize",
+            },
+        }
+
+        assert agent.can_handle("issue_comment", event_data) is False
+
+    def test_format_pr_summary_structure(self):
+        """Test _format_pr_summary generates correct markdown structure."""
+        from agents.pr_agent import PRAgent
+
+        agent = PRAgent(config={})
+
+        summary_data = {
+            "summary": "This PR adds a new feature",
+            "change_type": "Feature",
+            "key_changes": {
+                "added": ["New authentication module", "User login endpoint"],
+                "modified": ["Updated config file"],
+                "removed": ["Deprecated legacy auth"],
+            },
+            "files_affected": [
+                {
+                    "path": "src/auth.py",
+                    "description": "New authentication module",
+                    "change_type": "added",
+                },
+                {
+                    "path": "config.yml",
+                    "description": "Added auth settings",
+                    "change_type": "modified",
+                },
+            ],
+            "impact": {
+                "scope": "medium",
+                "description": "Adds authentication without breaking existing features",
+            },
+        }
+
+        result = agent._format_pr_summary(summary_data)
+
+        # Verify structure
+        assert "## 📋 Pull Request Summary" in result
+        assert "This PR adds a new feature" in result
+        assert "**Type:** ✨ Feature" in result
+        assert "## Changes" in result
+        assert "**✅ Added:**" in result
+        assert "**📝 Modified:**" in result
+        assert "**❌ Removed:**" in result
+        assert "## Files Affected" in result
+        assert "➕ `src/auth.py`" in result
+        assert "📝 `config.yml`" in result
+        assert "## Impact" in result
+        assert "🟡 **Scope:** Medium" in result
+
+    def test_format_pr_summary_change_types(self):
+        """Test that all change types have correct emojis."""
+        from agents.pr_agent import PRAgent
+
+        agent = PRAgent(config={})
+
+        change_types = {
+            "Feature": "✨",
+            "Bugfix": "🐛",
+            "Refactor": "♻️",
+            "Documentation": "📚",
+            "Testing": "🧪",
+            "Mixed": "🔀",
+        }
+
+        for change_type, expected_emoji in change_types.items():
+            summary_data = {
+                "summary": "Test",
+                "change_type": change_type,
+                "key_changes": {},
+                "files_affected": [],
+                "impact": {},
+            }
+
+            result = agent._format_pr_summary(summary_data)
+            assert f"**Type:** {expected_emoji} {change_type}" in result
+
+    def test_config_has_auto_summary_settings(self):
+        """Verify config.yml has auto_summary configuration."""
+        config_path = os.path.join(
+            os.path.dirname(__file__), "..", "tools", "ai-review", "config.yml"
+        )
+
+        import yaml
+
+        with open(config_path) as f:
+            config = yaml.safe_load(f)
+
+        assert "agents" in config
+        assert "pr" in config["agents"]
+        assert "auto_summary" in config["agents"]["pr"]
+        assert "enabled" in config["agents"]["pr"]["auto_summary"]
+        assert "post_as_comment" in config["agents"]["pr"]["auto_summary"]
+
+
 if __name__ == "__main__":
     pytest.main([__file__, "-v"])