* feat: enhance Excalidraw MCP with advanced canvas toolkit features
- Rename skill to `excalidraw-skill` with expanded playbook and cheatsheet.
- Add new MCP tools for iterative refinement: `describe_scene` and `get_canvas_screenshot`.
- Implement layout tools (`align_elements`, `distribute_elements`) and `duplicate_elements`.
- Add file I/O support for `.excalidraw` JSON and image export (PNG/SVG).
- Introduce named snapshots for canvas state management.
- Add server-side element CRUD and WebSocket handlers for real-time sync.
- Normalize `points` format for arrows and lines.
* docs: update README with v2.0 features and official MCP comparison
* feat: implement arrow binding and edge-to-edge routing
* fix: enhance security with path sanitization and improve export error handling
* feat: add viewport control, design guide, and excalidraw.com URL export
* feat: enhance excalidraw.com export with proper scene formatting and labels
