A frontend cell task waits on the UX/UI design, but a dev/code subtask
delegated under it did not inherit the unresolved dependency, so the
developer became dispatchable and coded ahead of the design. Propagate
the parent cell task's still-unresolved dependencies onto the new subtask
on delegate, reusing the existing dependency model: the subtask is held by
list_pending(filter_by_dependencies=True) until the UX task is terminal.
