inject the app system banner so cloud stops 400ing into the ws fallback

the upstream silently requires the exact app prompt banner inside the system message; without it every cloud call returns 400 invalid request no matter what headers or auth we send, so everything fell into the local websocket agent. bisected live against upstream and verified the proxy now serves cloud 200s streaming and non-streaming. details in .dbg/ROOT-CAUSE-AND-STUDY.md (local only)