Mark-done now fires OnChildTerminalAsync, so ticking a chain child or the last open child
off by hand unblocks the successor and advances the parent. The hub's Mark-as menu and
update_task_status map onto MarkDone/Cancel(retire)/Enqueue/Reset; revert_merge uses the
new ReopenForReviewAsync (Done|WaitingForReview -> WaitingForReview).