feat: consolidate testdrive-jsui to capabilities and implement plugin self-declaration

## Major Changes
- Moved all testdrive-jsui assets from root to capabilities/testdrive-jsui/
- Consolidated directory structure: js/, static/css/, static/images/, static/templates/
- Implemented plugin self-declaration (get_plugin_source_dir, get_asset_paths)
- Removed hardcoded plugin discovery from rendering.py
- Updated all asset paths to be relative to capability root

## Architecture Improvements
- Single source of truth for all testdrive-jsui assets
- Plugin declares its own location (no hardcoded paths)
- Generic plugin discovery using hasattr check
- Clean separation: all JS in .js files, no code mixing
- Standalone capability ready for independent use

## Files Changed
- markitect/plugins/testdrive_jsui.py: Added self-declaration methods
- markitect/plugins/rendering.py: Removed hardcoded discovery
- capabilities/testdrive-jsui/README.md: Added standalone usage documentation
- Moved 17 asset files to consolidated structure
- Deleted obsolete /testdrive-jsui/ root directory

## Testing
- All 17 assets verified and working
- Tested via CLI: markitect md-render --engine testdrive-jsui
- Full document rendering successful

Prepares testdrive-jsui to become a git submodule with proper dependency management.

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

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

markitect/plugins/rendering.py

@@ -296,17 +296,21 @@ class RenderingEngineManager:
         return deployed_assets
 
     def _get_plugin_source_dir(self, engine_name: str) -> Optional[Path]:
-        """Get the source directory for a plugin."""
-        if engine_name == 'testdrive-jsui':
-            # Look for testdrive-jsui directory relative to current directory
-            candidates = [
-                Path('testdrive-jsui'),
-                Path(__file__).parent.parent.parent / 'testdrive-jsui',
-                Path('.') / 'testdrive-jsui'
-            ]
+        """
+        Get the source directory for a plugin.
+        Uses plugin self-declaration if available, no hardcoded paths.
+        """
+        engine = self.get_engine(engine_name)
+        if not engine:
+            return None
 
-            for candidate in candidates:
-                if candidate.exists() and candidate.is_dir():
-                    return candidate
+        # Use plugin's self-declared source directory if available
+        if hasattr(engine, 'get_plugin_source_dir'):
+            try:
+                source_dir = engine.get_plugin_source_dir()
+                if source_dir and source_dir.exists():
+                    return source_dir
+            except Exception as e:
+                print(f"⚠️ Error getting plugin source dir for {engine_name}: {e}")
 
         return None