plan: align AI chat protocol with Vercel/ai SDK

Agent-Logs-Url: https://github.com/objectstack-ai/spec/sessions/2515d700-01d1-4303-8678-41f0ce9674da

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot

packages/spec/package.json

@@ -123,6 +123,7 @@
     "vitest": "^4.1.2"
   },
   "dependencies": {
+    "ai": "^6.0.142",
     "zod": "^4.3.6"
   },
   "engines": {