feat: complete CLI integration with plugin system

**CLI Integration:**
- Added --engine parameter to md-render command
- Default engine selection: testdrive-jsui for edit/insert, standard for view
- Graceful fallback to standard rendering when plugin unavailable
- Engine validation and mode compatibility checking

**Plugin Discovery:**
- Enhanced RenderingEngineManager with builtin plugin registration
- Automatic discovery and registration of testdrive-jsui engine
- Support for both plugin system discovery and direct registration

**Configuration Management:**
- Production-ready RenderingConfig for CLI usage
- Asset deployment to _markitect/plugins/ structure
- Configurable asset base URLs and deployment strategies

**Testing Infrastructure:**
- Comprehensive test suite for plugin discovery
- CLI integration testing without Click framework dependencies
- Complete scenario testing (default, explicit, fallback, unknown engines)
- Integration verification scripts

**Documentation:**
- Complete PLUGIN_SYSTEM.md documentation
- Architecture overview and development workflows
- JavaScript-first development guide
- Asset management and deployment strategies
- CLI usage examples and troubleshooting guide

**Key Features:**
- `markitect md-render --edit` now uses testdrive-jsui by default
- `markitect md-render --engine testdrive-jsui --edit` for explicit selection
- `markitect md-render --engine standard --edit` for legacy behavior
- Automatic fallback with user-friendly error messages

This completes the plugin infrastructure implementation, enabling
independent JavaScript development with seamless CLI integration.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

test_plugin_discovery.py

@@ -0,0 +1,80 @@
+#!/usr/bin/env python3
+"""
+Test plugin discovery and basic integration
+"""
+
+import sys
+from pathlib import Path
+
+# Add current directory to path for imports
+sys.path.insert(0, str(Path(__file__).parent))
+
+def test_plugin_discovery():
+    """Test that the plugin system can discover testdrive-jsui."""
+    print("🔍 Testing Plugin Discovery")
+    print("=" * 50)
+
+    try:
+        # Test basic plugin imports
+        print("1️⃣ Testing plugin imports...")
+        from markitect.plugins import PluginManager, RenderingEngineManager, RenderingConfig
+        print("   ✅ Plugin classes imported successfully")
+
+        # Test plugin manager initialization
+        print("\n2️⃣ Testing plugin manager initialization...")
+        plugin_manager = PluginManager()
+        print("   ✅ Plugin manager initialized")
+
+        # Test rendering engine manager
+        print("\n3️⃣ Testing rendering engine manager...")
+        rendering_manager = RenderingEngineManager(plugin_manager)
+        print("   ✅ Rendering engine manager initialized")
+
+        # List available engines
+        print("\n4️⃣ Testing engine discovery...")
+        engines = rendering_manager.list_engines()
+        print(f"   📋 Found engines: {engines}")
+
+        # Test testdrive-jsui specifically
+        print("\n5️⃣ Testing testdrive-jsui engine...")
+        engine = rendering_manager.get_engine('testdrive-jsui')
+        if engine:
+            print(f"   ✅ TestDrive JSUI engine found!")
+            print(f"   📝 Name: {engine.metadata.name}")
+            print(f"   📄 Description: {engine.metadata.description}")
+            print(f"   🎯 Supported modes: {engine.get_supported_modes()}")
+
+            # Test render capabilities
+            print("\n6️⃣ Testing render capabilities...")
+            test_content = "# Test\n\nThis is a test document."
+
+            config = RenderingConfig(
+                asset_base_url="_markitect",
+                development_mode=False,
+                output_directory=Path("/tmp")
+            )
+
+            html_output = engine.render_document(test_content, 'edit', config)
+            print(f"   ✅ Rendered HTML ({len(html_output):,} characters)")
+
+            # Save test output
+            test_file = Path("/tmp/plugin_discovery_test.html")
+            test_file.write_text(html_output)
+            print(f"   📄 Saved test output: {test_file}")
+
+        else:
+            print("   ❌ TestDrive JSUI engine not found")
+            return False
+
+        print(f"\n🎉 Plugin discovery test completed successfully!")
+        return True
+
+    except Exception as e:
+        print(f"❌ Plugin discovery test failed: {e}")
+        import traceback
+        traceback.print_exc()
+        return False
+
+if __name__ == "__main__":
+    success = test_plugin_discovery()
+    sys.exit(0 if success else 1)