Fire-and-forget background subagents that re-inject results as a new turn (unreleased)

What this changes for operators

• delegate_task(background=true) (main) dispatches an async subagent and re-injects its result as a new turn, with /stop and /agents as the control surface and a max_async_children cap. The same week removed the default 600s subagent timeout, so runaway detection now rests on heartbeat staleness alone. Changes the unit of work and the receipt boundary.