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>

markitect/plugins/builtin/markdown_commands.py

@@ -2230,6 +2230,19 @@ def md_render_command(ctx, input_file, output, theme, css, edit, insert, engine,
                     # Render using plugin
                     html_content = rendering_engine.render_document(content, 'edit', render_config)
 
+                    # Deploy plugin assets
+                    if not silent:
+                        click.echo(f"📦 Deploying assets for engine '{engine}'...")
+
+                    deployed_assets = rendering_manager.deploy_engine_assets(engine, render_config)
+
+                    if verbose and deployed_assets:
+                        total_assets = sum(len(files) for files in deployed_assets.values() if isinstance(files, list))
+                        click.echo(f"   📄 Deployed {total_assets} asset files")
+                        for asset_type, files in deployed_assets.items():
+                            if isinstance(files, list) and files:
+                                click.echo(f"   {asset_type}: {len(files)} files")
+
                     # Write output
                     output_path.write_text(html_content, encoding='utf-8')
                     result = True
@@ -2283,6 +2296,19 @@ def md_render_command(ctx, input_file, output, theme, css, edit, insert, engine,
                         if not silent:
                             click.echo(f"ℹ️ Engine '{engine}' doesn't support insert mode, using edit mode instead")
 
+                    # Deploy plugin assets
+                    if not silent:
+                        click.echo(f"📦 Deploying assets for engine '{engine}'...")
+
+                    deployed_assets = rendering_manager.deploy_engine_assets(engine, render_config)
+
+                    if verbose and deployed_assets:
+                        total_assets = sum(len(files) for files in deployed_assets.values() if isinstance(files, list))
+                        click.echo(f"   📄 Deployed {total_assets} asset files")
+                        for asset_type, files in deployed_assets.items():
+                            if isinstance(files, list) and files:
+                                click.echo(f"   {asset_type}: {len(files)} files")
+
                     # Write output
                     output_path.write_text(html_content, encoding='utf-8')
                     result = True