The ACP harness is single-relay but serverless workspaces pass a
comma-separated relay LIST (wss://a,wss://b,...). connect_with_mode fed
the raw comma string to connect_async, which failed DNS lookup
("nodename nor servname provided") and crashed the agent before it ever
subscribed — so it never saw mentions and never responded.
Split to the first (primary) relay, matching the desktop live-WS
convention. Regression-covered by the live agent test now using a
comma-separated RELAY_URL.