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

tsconfig.app.json

@@ -15,7 +15,11 @@
      */
     "allowJs": true,
     "checkJs": true,
-    "moduleDetection": "force"
+    "moduleDetection": "force",
+    "paths": {
+      "@agor/types": ["./packages/types/index.ts"],
+      "@agor/types/*": ["./packages/types/*.ts"]
+    }
   },
-  "include": ["src/**/*.ts", "src/**/*.js", "src/**/*.svelte"]
+  "include": ["src/**/*.ts", "src/**/*.js", "src/**/*.svelte", "packages/types/**/*.ts"]
 }