Spec: AI SDK chat

Chat is not in the default clone.

npx skills add vercel/ai@ai-sdk

Then follow .agents/skills/add-ai-sdk/SKILL.md.

Rules

• Keys stay on the server
• Use streamText / generateText
• Gate /api/chat with the existing Auth.js session
• Postgres for history if needed; Redis only if the user asked
• Model via env, not a hardcoded vendor