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

.gitignore

@@ -1,5 +1,8 @@
 /target
+crates/*/target
 Cargo.lock
 *.swp
 *.swo
 .DS_Store
+opencode.jsonc
+.opencode