feat: complete asset deployment for plugin engines

**Asset Deployment Infrastructure:**
- Enhanced RenderingEngineManager with complete asset deployment
- Automatic plugin source directory discovery and asset copying
- File system operations with proper directory structure preservation
- Comprehensive error handling for missing assets

**CLI Integration:**
- Automatic asset deployment when using plugin engines
- Verbose output showing deployment progress and statistics
- Asset verification and accessibility validation
- Production-ready deployment to _markitect/plugins/ structure

**TestDrive JSUI Assets:**
- Complete CSS asset suite (editor, controls, GitHub theme)
- Placeholder image assets for testing deployment
- Proper asset organization following plugin conventions
- All 18 assets now deployed correctly

**Testing Infrastructure:**
- Comprehensive asset deployment testing
- CLI integration verification with asset shipping
- File existence and accessibility validation
- Complete directory structure verification

**Key Features:**
- Assets deployed to `_markitect/plugins/testdrive-jsui/` when using --edit
- HTML references match deployed asset locations
- 18 total assets: 12 JS, 3 CSS, 3 images
- Automatic deployment without --ship-assets flag needed
- Clean separation of development vs production asset handling

**Example Output:**
```
🎯 Using rendering engine: testdrive-jsui (supports: edit, view)
📦 Deploying assets for engine 'testdrive-jsui'...
📄 Deployed 18 asset files
   js: 12 files
   css: 3 files
   images: 3 files
✅ Rendered with INTERACTIVE editing mode
```

This fixes the "HTML assets not found" issue when using explicit
output directories with plugin engines. All plugin assets are now
properly deployed and accessible.

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

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

docs/PLUGIN_SYSTEM.md

@@ -266,6 +266,46 @@ config = RenderingConfig(
 3. **Cache Management**: Asset versioning and cache control
 4. **Error Handling**: Fallback for missing assets
 
+### Asset Deployment Process
+
+When using CLI with plugin engines, assets are automatically deployed:
+
+```bash
+# Assets are deployed to output directory when using plugin engines
+markitect md-render --edit document.md --output /path/to/output.html
+
+# Output structure:
+# /path/to/
+# ├── output.html
+# └── _markitect/
+#     └── plugins/
+#         └── testdrive-jsui/
+#             ├── static/
+#             │   ├── js/          # 12 JavaScript files
+#             │   └── css/         # 3 CSS files
+#             └── images/          # 3 image files
+```
+
+The deployment process:
+
+1. **Plugin Discovery**: Engine identified (default: testdrive-jsui for edit mode)
+2. **Asset Analysis**: Required assets determined from `get_required_assets()`
+3. **Source Resolution**: Plugin source directory located
+4. **File Copying**: Assets copied with directory structure preservation
+5. **URL Generation**: HTML references generated with correct paths
+6. **Verification**: Asset accessibility validated
+
+Example output:
+```
+🎯 Using rendering engine: testdrive-jsui (supports: edit, view)
+📦 Deploying assets for engine 'testdrive-jsui'...
+📄 Deployed 18 asset files
+   js: 12 files
+   css: 3 files
+   images: 3 files
+✅ Rendered with INTERACTIVE editing mode to: output.html
+```
+
 ## Configuration Interface
 
 ### Python → JavaScript Data Transfer