Warn when supervisor refuses a task request without changing dispatch behavior. Both approved refusal branches now emit exactly one warning: active-command skips and the _task_queue-unavailable fall-through. On the pre-fix tree neither branch emitted a warning, so a request could be refused without an operator-visible log line. This addresses defect (a)'s invisibility: the supervisor refused the request and said nothing. It does not change queueing, skipping, or no-flag dispatch behavior.