Deliver Dispatch Results

Deliver a handed-off specialist's answer as its own turn.

The head no longer waits out a slow dispatch: once the specialist has started, the job is handed off and the turn ends with an acknowledgement (DispatchHandoffEvent). This is what keeps that promise. The surface polls here while an answer is outstanding; the poll is a 204 until a job reaches a terminal status, and then this streams a turn that narrates it.

No user message is created — the person did not say anything. The finished job is the turn's input, the same way /tool-result resumes from a ToolMessage rather than fresh text.

Claiming is atomic and exactly-once (consume_orphaned_sub_agent_jobs), so this racing a message the person sent at the same moment cannot deliver the same answer twice: whichever gets there first has it.