Stream a Y2 Agent response

Streams the preconfigured Y2 Agent with the same tools, entitlements, chat-credit budget, and API-key rate limits as the Y2 app copilot. This is not a general multi-model API. v1 callers cannot choose models, use onboarding mode, or attach files. Transcript history supplies user/assistant context but cannot override the fixed system instructions. v1 requires an API key and does not support x402. Both Agent Y2 routes share a throttle of 5 requests/minute and 100/day per key, plus 10 requests/minute and 250/day per user or workspace. A `402` indicates subscription, chat-credit, or upstream-provider credit exhaustion, not an x402 challenge.