Root cause: user-level hermes-gateway.service was running the same
-p default gateway as the system-level hermes-gateway@default.service,
causing the two to SIGTERM each other every ~5 minutes. This killed
in-flight Hermes API requests with 'Remote end closed connection'.
Fix: disabled the redundant user-level service.
Also: simplified webhook to just forward raw message content (no
system prompt, no ACK, no max_tokens overrides). Removed unused
_get_ack method.