fix(worker,ui): route details-pane task delete through worker to advance blocked parents

Deleting a child from the details pane hard-deleted straight from the UI
process via TaskRepository, bypassing TaskStateService.TryAdvanceParentAsync
entirely. Deleting the last child of a WaitingForChildren parent that way
left it wedged forever. WorkerHub.DeleteTask now mirrors the MCP delete_task
tool (running-task guard, FK-friendly error, advance-parent call), and the
UI goes through it.

TryAdvanceParentAsync also short-circuited when zero children remained,
treating "no children left" as "nothing to evaluate" instead of "all done" -
removed the early return so an empty child list (vacuously) counts as all
terminal.

docs/explore-notes/worker-task-pipeline.md

@@ -50,6 +50,9 @@ How a task moves Queued → Running → terminal, across `src/ClaudeDo.Worker`
    - **Done** CompleteAsync (Running → Done) — top-level success.
    - **WaitingForReview** SubmitForReviewAsync (Running → WaitingForReview) — review gate.
    - **WaitingForChildren** SubmitForChildrenAsync (Running → WaitingForChildren) — blocks on children.
+     Advances to WaitingForReview via `TryAdvanceParentAsync` once every remaining child is
+     terminal (Done/Failed/Cancelled) — including zero children left, e.g. after the last child
+     is deleted (`WorkerHub.DeleteTask` / `ExternalMcpService.DeleteTask` both call it).
    - **Failed** FailAsync (Running/Queued → Failed).
    - **Cancelled** CancelAsync (Running/Queued/WaitingForReview/WaitingForChildren → Cancelled).