fix(subagent): confirm steering request admission

docs/core-data-structures/core.md

@@ -560,6 +560,8 @@ interface CancelOptions {
 }
 ```
 
+`SteeringReceipt.outcome` always resolves. `admitted` identifies the turn and step whose immutable request history contains that exact message; `rejected` means lifecycle or terminal policy discarded it first. Synchronous input validation still throws from `steer()`.
+
 ```ts type-equiv
 /** Stable runtime cause accepted by {@link Agent.cancel}. */
 type AgentCancelCause =
@@ -661,26 +663,18 @@ interface Agent {
   followup(message: UserMessage): void
 
   /**
-   * Submit steering during prompt admission or an open turn — the
-   * `next-step`/wakeup preset of {@link send}. It stages for the next steering
-   * checkpoint before a request or stop decision. If the activity fails before
-   * that boundary, the remainder stays staged without waking the agent; retry
-   * or a later prompt takes it. Outside that window steering falls back to a
-   * woken follow-up turn, while cancellation or disposal may discard pending
-   * steering.
+   * Submit steering with a message-owned admission receipt — the
+   * `next-step`/wakeup preset of {@link send}. During prompt admission or an
+   * open turn, the message waits in the steering FIFO until a committed step
+   * snapshots it; outside that window it enters the ordinary queued FIFO. The
+   * receipt resolves `admitted` only after the message joins that step's
+   * immutable request history, or `rejected` when terminal policy,
+   * cancellation, or disposal discards it first. A non-terminal turn close may
+   * leave it staged for a later admitted prompt without settling the receipt.
    * @param message - identified steering content and its producer provenance.
+   * @returns the receipt for this exact message's eventual admission outcome.
    */
-  steer(message: UserMessage): void
-
-  /**
-   * Atomically submit steering only while the current step still owns its final
-   * drain. Returns `false` without accepting the message during admission,
-   * between steps, or after the final per-step drain has begun. Cancellation or
-   * disposal may still discard previously accepted steering.
-   * @param message - identified steering content and its producer provenance.
-   * @returns whether the message entered the current step.
-   */
-  trySteer?(message: UserMessage): boolean
+  steer(message: UserMessage): SteeringReceipt
 
   /**
    * Append model-facing context without running the model — the