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>

markitect/plugins/rendering.py

@@ -183,7 +183,7 @@ class RenderingEngineManager:
 
     def _discover_rendering_engines(self):
         """Discover rendering engine plugins."""
-        # Get all plugins from the main plugin manager
+        # First, try to load plugins from main plugin manager
         all_plugins = self.plugin_manager.discover_plugins()
 
         for plugin_name, plugin_info in all_plugins.items():
@@ -197,6 +197,22 @@ class RenderingEngineManager:
                 except Exception as e:
                     print(f"⚠️ Failed to load rendering engine {plugin_name}: {e}")
 
+        # Additionally, try to directly import and register known rendering engines
+        self._register_builtin_rendering_engines()
+
+    def _register_builtin_rendering_engines(self):
+        """Register built-in rendering engines directly."""
+        try:
+            # Import and register testdrive-jsui engine
+            from .testdrive_jsui import TestDriveJSUIEngine
+            engine = TestDriveJSUIEngine()
+            self._engines[engine.metadata.name] = engine
+            print(f"✅ Registered built-in rendering engine: {engine.metadata.name}")
+        except ImportError as e:
+            print(f"⚠️ Could not import testdrive-jsui engine: {e}")
+        except Exception as e:
+            print(f"⚠️ Failed to register testdrive-jsui engine: {e}")
+
     def get_engine(self, name: str) -> Optional[RenderingEnginePlugin]:
         """Get a rendering engine by name."""
         return self._engines.get(name)