fix(subagent): confirm steering request admission

packages/core/agent/src/types.ts

@@ -58,6 +58,20 @@ export type InboxAction =
 /** Result of applying an inbox action at the synchronous ownership boundary. */
 export type InboxActionResult = 'applied' | 'not-found'
 
+/** Final admission outcome for one call to {@link Agent.steer}. */
+export type SteeringOutcome =
+  | { readonly status: 'admitted'; readonly turn: number; readonly step: number }
+  | { readonly status: 'rejected' }
+
+/**
+ * Message-owned steering admission receipt. The outcome promise always
+ * resolves: synchronous input validation still throws from {@link Agent.steer},
+ * while lifecycle policy reports non-admission as `rejected`.
+ */
+export interface SteeringReceipt {
+  readonly outcome: Promise<SteeringOutcome>
+}
+
 /**
  * Options for the unified {@link Agent.send} primitive over the
  * (`target` × `wakeup`) matrix. Named presets: {@link Agent.followup}
@@ -225,26 +239,18 @@ export 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