Add agent drawing commands, session management, and MCP write tools

- Add CreateDrawingElement, UpdateDrawingElement, DeleteDrawingElement,
  ClearDrawingElements to WebSocket protocol and MCP bridge
- Add multi-session/tab support with SessionStore shared state
- Add bidirectional agent-GUI sync via broadcast + mpsc channels
- Update README with correct OpenCode MCP config format and new tools
- Fix dead code warning, clean up gitignore

examples/mcp-configs/opencode.json

@@ -1,9 +0,0 @@
-{
-  "$schema": "https://opencode.ai/config.schema.json",
-  "mcpServers": {
-    "agcanvas": {
-      "command": "agcanvas-mcp",
-      "args": ["--port", "9876"]
-    }
-  }
-}