feat: @agor/types package + BackendAdapter + TauriAdapter + ElectrobunAdapter

- packages/types/: shared type definitions (agent, project, btmsg, bttask,
  health, settings, protocol, backend interface)
- BackendAdapter: capability-flagged interface, compile-time selected
- TauriAdapter: wraps Tauri invoke/listen
- ElectrobunAdapter: wraps Electrobun RPC
- src/lib/backend/backend.ts: adapter singleton + setBackendForTesting()
- pnpm-workspace.yaml: workspace setup

package.json

@@ -3,6 +3,9 @@
   "private": true,
   "version": "0.1.0",
   "type": "module",
+  "workspaces": [
+    "packages/*"
+  ],
   "scripts": {
     "dev": "vite",
     "prebuild": "cp node_modules/pdfjs-dist/build/pdf.worker.min.mjs public/pdf.worker.min.mjs",