fix(subagent): confirm steering request admission

packages/subagent/subagent/README.md

@@ -45,7 +45,7 @@ Start-time features are advertised in `provider.capabilities` because the servic
 - `toolFilter` — apply the requested child tool restriction.
 - `persona` — apply a per-child persona.
 
-Runtime features are optional methods whose presence is the capability check: `SubagentRun.steer?` delivers strictly to the actively running child turn (it throws rather than queueing when the child is not running), and `SubagentProvider.resume?` reconstructs a persisted continuable child. A run represents one disposable activation, so it deliberately has no cold-resume operation — a disposed run cannot be reconstructed after restart.
+Runtime features are optional methods whose presence is the capability check: `SubagentRun.steer?` fulfills only after a request snapshot in the active child admits the message and rejects rather than queueing an untracked turn, while `SubagentProvider.resume?` reconstructs a persisted continuable child. A run represents one disposable activation, so it deliberately has no cold-resume operation — a disposed run cannot be reconstructed after restart.
 
 ## The durable descriptor
 

packages/subagent/subagent/README.zh.md

@@ -42,12 +42,12 @@ subagent seam 允许一个 agent（智能体）通过具名提供方把工作委
 - `toolFilter`：应用请求的子 agent 工具限制；
 - `persona`：应用每个子 agent 独立的 persona。
 
+运行时功能通过可选方法是否存在来检查能力：`SubagentRun.steer?` 只有在活跃子 agent 的请求 snapshot 接纳消息后才会兑现，并会拒绝而非排队一个未跟踪轮次；`SubagentProvider.resume?` 则重建已持久化且可继续的子 agent。一次运行表示一个可 dispose（资源释放）的 activation，因此刻意不提供冷恢复操作；已释放的运行无法在重启后重建。
+
 ## 委派深度
 
 该 seam 拥有实现和消费方共享的深度词汇：`AgentOptions.subagentDepth` 声明、`assertSubagentMaxDepth` 和 `delegationDepthOf(agent)`。持久化的 `SessionHeader.delegationDepth` 具有权威性且单调：运行时选项可以加深计数，但绝不能降低它，因此恢复后的子 agent 不会被重新计为顶层。
 
-运行时功能是 `SubagentRun` 上的可选方法：`sendMessage?` 可对正在运行的子 agent 进行 steering（中途引导），`resume?` 则异步创建延续运行。方法是否存在就是能力检查。
-
 `inheritsParentContext` 只用于描述，不能强制执行。它仅说明子 agent 是否能看到父级已完成的对话历史（`fork` 可以；`spawn` 和 ACP 不可以），不表示是否继承工具、服务或权限。
 
 ## 所有权与生命周期

packages/subagent/subagent/src/types.ts

@@ -28,7 +28,7 @@ export function SubagentRunId(id: string): SubagentRunId {
  * {@link SubagentProvider.start}: a request that needs a capability the chosen provider lacks
  * is rejected with a typed error rather than accepted-then-ignored (the "fail loud, no silent
  * degradation" rule). These static flags cover features needed before a run exists; runtime
- * capabilities are optional methods whose presence is the capability — strict live steering
+ * capabilities are optional methods whose presence is the capability — confirmed live steering
  * is {@link SubagentRun.steer} and persisted cold resume is {@link SubagentProvider.resume}. Each
  * flag corresponds one-to-one to a {@link SubagentStartRequest} option: `depthLimit` to
  * `maxDepth`; the other names match.
@@ -221,19 +221,16 @@ export interface SubagentRun {
    */
   dispose(): Promise<void>
   /**
-   * OPTIONAL (strict live-steering capability): deliver additional content to
-   * the actively running child turn. STRICT means delivery joins the observed
-   * turn or fails — the implementation must synchronously verify, with no
-   * asynchronous boundary before delivery, that the child is running and its
-   * turn can still record the message, and must not fall back to a queue path
-   * that could start a new, untracked turn or silently drop the message after
-   * this run has settled. Throws when delivery cannot join the turn. A run
-   * represents one disposable activation, so it has no cold-resume operation;
-   * resuming a settled child goes through {@link SubagentProvider.resume}.
-   * `source` is retained on the child's logged steering message without
-   * changing its user role in model history.
+   * OPTIONAL (confirmed live-steering capability): submit additional content
+   * to the active child and fulfill only after a committed request snapshot
+   * admits it. Rejects when terminal policy, cancellation, disposal, or a lost
+   * settlement race prevents admission; it never falls through to a queued
+   * untracked turn or cold resume. A run represents one disposable activation,
+   * so resuming a settled child goes through {@link SubagentProvider.resume}.
+   * `source` is retained on the admitted steering message without changing its
+   * user role in model history.
    */
-  steer?(content: ContentBlock[], source: MessageSource): void
+  steer?(content: ContentBlock[], source: MessageSource): Promise<void>
 }
 
 /**