update_task_status Idle/Done, WorkerHub.SetTaskStatus, and the UI's Mark
Done/Cancelled menu items could all force a Running task's status without
going through TaskStateService's guards, leaving its CLI process untracked
and its worktree writes silently dropped. Idle now routes through the
already-guarded TaskStateService.ResetToIdleAsync instead of the repo's
unconditional ResetToManualAsync; Done and SetTaskStatus explicitly reject
Running; the context menu disables both entries with a tooltip while
running. TaskRunner also now logs a warning whenever a terminal transition
(SubmitForChildren/SubmitForReview/Complete/Fail) is rejected instead of
silently discarding the run result. DeleteTask's MCP path gets the same
friendly foreign-key message WorkerHub.DeleteTask already had for a task
with children.
