docs: add Cursor Cloud specific instructions (mock server + sessionId gotcha)

[NeilJo-GY]

Summary

Adds a ## Cursor Cloud specific instructions section to AGENTS.md capturing durable, non-obvious notes for running this server in a cloud dev environment:

• Zero-dependency repo; npm run dev starts the mock-provider server on port 3721 with no key or bridge.
• Headless smoke-test flow: POST /v1/session/start returns a sessionId that is not implicitly active — POST /v1/input/text must echo that sessionId in the body, otherwise it returns {"error":"Session not found: undefined"}. GET /v1/status takes ?sessionId= as a query param.
• Live2D/VRM/HeyGen providers require external SDKs/bridges/keys and are out of scope for the default setup.

Testing

Docs-only change. Verified the documented flow during setup: /health → {ok:true}, session start → sessionId, and text input with that id returned the mock response "Mock avatar received: …".