Confirmed via docs/open.md (Korrektheits-Review 2026-06-09): TaskStateService.CancelAsync
only flipped the target row to Cancelled - a Queued/Running child kept going and could
still commit into its worktree after the parent was gone.
CancelAsync now also cancels the task's own non-terminal children (Idle/Queued/Running/
WaitingForReview/WaitingForChildren) in the same pass, clearing BlockedByTaskId so no
successor is left wedged, and broadcasts TaskUpdated for each.