Merge PR #43: perf/fix(relay) take herdr calls off the event loop, real key grammar

Wraps blocking run_herdr calls in asyncio.to_thread so a slow herdr call
(or SSH timeout to a remote) no longer stalls every connected client.
Also fixes the key grammar to match herdr's real bindings.

Conflict resolution: combined #43's asyncio.to_thread for agent_prompt with
#41's request_id echo — kept both. Thanks @nickrunning!