Relay operations

AGENTPEER_RELAY_SECRET authenticates loopback/compose-only API↔relay calls. Rotate it by placing the same new random value in API and relay environments and restarting both; never place the internal endpoints behind Caddy. Relay Redis uses agentpeer_relay and only inbox:*, relayconn:*, and presence:*.

Restart drill: stop the relay, leave Redis running, restart it, and confirm clients reconnect with exponential backoff and their last inbox stream ID. Delivery resumes from the inbox without a second live path. A cursor older than the retained first ID receives resync_required.

Redis outage: /healthz remains process liveness, /readyz returns 503 with redis: unavailable; authentication may succeed through the API, but XADD failure sends nack unavailable and nothing is delivered live. Restore Redis and readiness recovers. Shutdown closes connections with 1012; revocation uses 4403.