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

vite.config.ts

@@ -1,8 +1,14 @@
 import { defineConfig } from 'vite'
 import { svelte } from '@sveltejs/vite-plugin-svelte'
+import path from 'node:path'
 
 export default defineConfig({
   plugins: [svelte()],
+  resolve: {
+    alias: {
+      '@agor/types': path.resolve(__dirname, 'packages/types/index.ts'),
+    },
+  },
   server: {
     port: 9710,
     strictPort: true,