fix: address codex review round 3

- Strict steer additionally requires an OPEN STEP: between steps the
  loop may be awaiting its continuation/turn-stop checkpoints, where
  pending steering was already folded and a terminal stop discards a
  later arrival. A message accepted during an open step is drained and
  recorded at that step's settlement before any terminal decision, so
  the acknowledged-then-discarded window is closed. New keyless test
  holds agent/turn-stop open and pins the rejection.
- tool-subagent-control README: distinguish synchronous not-delivered
  errors from started-Task failures (unknown/foreign/descriptor-less
  ids settle the started Task as failed), and drop the claim that the
  completion notice carries the child's response.

packages/subagent/subagent-inprocess/README.md

@@ -30,7 +30,7 @@ The required request signal covers both startup and the live run. Before publica
 
 After fulfillment, the caller owns the run. Provider-plugin unload does not revoke it. `dispose()` removes the live abort listener, records cancellation, and delegates to the returned `AgentHandle.dispose()`, whose memoized quiescence transaction stops the loop, removes the agent and session, and unwinds scoped registrations. Cancellation owns every non-completed in-flight outcome and reports `aborted`; an already-completed turn remains completed.
 
-Runs expose the strict `steer` capability: the synchronous checks and the `Agent.steer()` call share one frame, so delivery joins the observed turn or throws. Delivery requires `AgentStatus.running`, an open turn in the child log (status stays `running` through a closed turn's durability flush, where the loop would strand the message), and no committed structured capture (whose terminal stop makes the loop discard late steering). The Agent-level idle fallback (queue and start a new turn) is deliberately not reachable through the run — that would start an untracked turn after the run's result was read.
+Runs expose the strict `steer` capability: the synchronous checks and the `Agent.steer()` call share one frame, so delivery joins the observed turn or throws. Delivery requires `AgentStatus.running`, an open turn in the child log (status stays `running` through a closed turn's durability flush, where the loop would strand the message), an open step (between steps the loop may sit at its continuation/turn-stop checkpoints, where steering was already folded and a terminal stop discards a later arrival; a message accepted during an open step is recorded at that step's settlement before any terminal decision), and no committed structured capture (whose terminal stop makes the loop discard late steering). The Agent-level idle fallback (queue and start a new turn) is deliberately not reachable through the run — that would start an untracked turn after the run's result was read.
 
 ## Spawn and fork inputs