- extend brightstaff llm_chat_inner to extract X-Session-Id, check the
session cache before routing, and cache the result afterward — same
pattern as routing_service.rs
- replace old urllib-based demo with a real FastAPI research agent that
runs 3 independent tool-calling tasks with alternating intents so
Plano routes to different models; demo.py is a pure httpx client that
shows the routing trace side-by-side with and without session pinning