feat(app): activate plugin registry with Claude provider adapter

Create plugins/index.ts bootstrap and claude-provider.ts implementing
the AIProviderAdapter interface from @aiui/core. The Claude provider
wraps the existing proxy streaming logic as an async generator. Plugin
initialization is called in main.ts before app mount.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/app/src/plugins/index.ts

@@ -0,0 +1,26 @@
+import { registerPlugin } from '@aiui/core'
+
+export async function initializePlugins(): Promise<void> {
+  // Register built-in AI provider
+  const { claudeProvider } = await import('./claude-provider')
+  registerPlugin(claudeProvider)
+  await claudeProvider.init({
+    settings: {
+      get: () => undefined,
+      set: () => {},
+    },
+    events: {
+      emit: () => {},
+      on: () => () => {},
+    },
+    logger: {
+      info: console.info.bind(console, '[AIUI plugin]'),
+      warn: console.warn.bind(console, '[AIUI plugin]'),
+      error: console.error.bind(console, '[AIUI plugin]'),
+    },
+  })
+
+  if (import.meta.env.DEV) {
+    console.log('[AIUI] Plugins initialized:', claudeProvider.id)
+  }
+}