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_cli_simple.py

@@ -0,0 +1,121 @@
+#!/usr/bin/env python3
+"""
+Test CLI plugin integration by directly calling the core logic
+"""
+
+import sys
+from pathlib import Path
+
+# Add current directory to path for imports
+sys.path.insert(0, str(Path(__file__).parent))
+
+def test_cli_integration():
+    """Test CLI integration logic without Click framework."""
+
+    print("🧪 Testing CLI Plugin Integration Logic")
+    print("=" * 50)
+
+    try:
+        # Import required components
+        from markitect.plugins import PluginManager, RenderingEngineManager, RenderingConfig
+
+        # Test input
+        input_file = "test_cli_plugin.md"
+        output_file = "/tmp/test_cli_integration.html"
+
+        if not Path(input_file).exists():
+            print(f"❌ Test file {input_file} not found")
+            return False
+
+        # Read markdown content
+        content = Path(input_file).read_text(encoding='utf-8')
+        print(f"📄 Read test content ({len(content)} characters)")
+
+        # Test 1: Plugin system initialization
+        print("\n1️⃣ Initializing plugin system...")
+        plugin_manager = PluginManager()
+        rendering_manager = RenderingEngineManager(plugin_manager)
+        print("   ✅ Plugin system initialized")
+
+        # Test 2: Engine selection logic (same as CLI)
+        engine_name = None
+        edit_mode = True
+
+        # Default engine selection (copied from CLI logic)
+        if engine_name is None:
+            if edit_mode:
+                engine_name = 'testdrive-jsui'  # Default to testdrive-jsui for interactive modes
+            else:
+                engine_name = 'standard'  # Use standard CleanDocumentManager for non-interactive
+
+        print(f"   🎯 Selected engine: {engine_name}")
+
+        # Test 3: Engine loading
+        print(f"\n2️⃣ Loading rendering engine '{engine_name}'...")
+        rendering_engine = rendering_manager.get_engine(engine_name)
+
+        if rendering_engine is None:
+            print(f"   ❌ Rendering engine '{engine_name}' not found")
+            return False
+
+        print(f"   ✅ Engine loaded: {rendering_engine.metadata.name}")
+        print(f"   📝 Description: {rendering_engine.metadata.description}")
+        print(f"   🎯 Supported modes: {rendering_engine.get_supported_modes()}")
+
+        # Test 4: Mode validation
+        current_mode = 'edit'
+        if not rendering_engine.validate_mode(current_mode):
+            print(f"   ❌ Engine doesn't support mode '{current_mode}'")
+            return False
+
+        print(f"   ✅ Mode '{current_mode}' is supported")
+
+        # Test 5: Rendering configuration
+        print(f"\n3️⃣ Setting up rendering configuration...")
+        render_config = RenderingConfig(
+            asset_base_url="_markitect",
+            development_mode=False,  # Production deployment for CLI usage
+            output_directory=Path(output_file).parent
+        )
+        print(f"   ✅ Configuration created")
+        print(f"   📁 Output directory: {render_config.output_directory}")
+        print(f"   🔗 Asset base URL: {render_config.asset_base_url}")
+
+        # Test 6: Document rendering
+        print(f"\n4️⃣ Rendering document...")
+        html_content = rendering_engine.render_document(content, current_mode, render_config)
+        print(f"   ✅ Document rendered ({len(html_content):,} characters)")
+
+        # Test 7: Output writing
+        print(f"\n5️⃣ Writing output file...")
+        Path(output_file).write_text(html_content, encoding='utf-8')
+        output_size = Path(output_file).stat().st_size
+        print(f"   ✅ Output written: {output_file} ({output_size:,} bytes)")
+
+        # Test 8: Verification
+        print(f"\n6️⃣ Verifying output...")
+        if Path(output_file).exists() and output_size > 0:
+            print(f"   ✅ Output file exists and has content")
+            print(f"   🌐 Open in browser: file://{Path(output_file).absolute()}")
+        else:
+            print(f"   ❌ Output file missing or empty")
+            return False
+
+        print(f"\n🎉 CLI integration test completed successfully!")
+        print(f"\n📊 Summary:")
+        print(f"   Engine: {engine_name}")
+        print(f"   Mode: {current_mode}")
+        print(f"   Input: {input_file} ({len(content)} chars)")
+        print(f"   Output: {output_file} ({output_size:,} bytes)")
+
+        return True
+
+    except Exception as e:
+        print(f"❌ CLI integration test failed: {e}")
+        import traceback
+        traceback.print_exc()
+        return False
+
+if __name__ == "__main__":
+    success = test_cli_integration()
+    sys.exit(0 if success else 1)