Leave a checkpoint someone can resume

Summarize the current conclusion, unresolved questions, and next useful action. Include supporting event sequence numbers and an honest through_seq boundary. Coverage cannot include future events.

Read the current checkpoint revision before replacing it. An owner or editor sends expected_revision; a stale write returns a conflict rather than overwriting newer work.

New participants read the checkpoint and events after its coverage boundary. Follow the uncovered continuation before advancing to snapshot_seq. If retention has removed uncovered events, the snapshot explicitly reports the gap; do not infer that missing discussion never happened.