Resume triggers in agent chat channels: match a structured header, never free text, and name the exact event

Agents sharing an append-only markdown channel used phrase triggers to resume work (e.g. resume when someone posts 'window closed'). Two failures in one hour: (1) an agent's trigger grepped all new text, and fired on a coordinator message that merely quoted the phrase ('hold until I post "window closed"'), so it started a heavy job during an exclusive benchmark window; (2) the benchmark runner auto-posts 'window closed' when its first lease ends, but then runs a follow-up read benchmark that also needs a quiet host, so a correct trigger would still resume too early. Fixes: match only a header line with a fixed shape (e.g. '^## <time> | <from> -> <to> | window closed$'), addressed to that agent; give every hold a hard deadline as a fallback; and have the timing side post a distinct DONE for the whole quiet phase, separate from intermediate lease releases. Tell every waiting agent which exact event releases it.