feat(subagent): deliver continuable child settlement to parents

A continuable child that stopped without reporting — an error, a token
ceiling, cancellation, teardown — left its parent nothing to act on.
The continuation manager now delivers an unconditional settlement
notice to the durable direct parent before releasing ownership, folding
consumed work (foldConsumedWork supersedes findLastMessageTurnEnd) so a
claimed-but-unrun prompt reads as aborted rather than completed, waking
an idle parent, steering a busy one, and never waking a closing tree.

docs/subsystems/subagent.zh.md

@@ -209,6 +209,27 @@ interface SubagentReportMessageSource {
 type SubagentReportDelivery = 'quiet' | 'wakeup'
 ```
 
+上报是 child 自己的选择，因此管理器还保有一份属于自己的记账：当驻留 Activation 结算时，它会向该 child 持久化的直接 parent 投递一条通知，说明该 epoch 如何结束，并携带其最终 assistant 内容。对每个调用方拿到过 id 的 child，这条投递都是无条件的；它发生在会让 parent 被判定为已结算的所有权释放之前，并通过与上报相同的唤醒准入记账到达驻留 parent。若 parent 自身所在的谱系已在拆卸中，这条通知会以不唤醒的方式送达，因为唤醒一个静息 Agent 是开启一个轮次，而不是排队等待工作。其来源信息使用一个独立的 kind，因此 transcript（文本记录）绝不会把运行时的记账呈现为 child 自己写下的内容。
+
+```ts type-equiv
+/**
+ * Durable attribution for the runtime's own account of a continuable child
+ * settling. Deliberately a different kind from
+ * {@link SubagentReportMessageSource}: a report is content the child chose,
+ * while this message is the manager stating what became of the child, and a
+ * transcript that merged them would credit the child with words it never wrote.
+ */
+interface SubagentSettledMessageSource {
+  readonly kind: 'subagent-settled'
+  /** A runtime account shown without expanding the row (`notice` context form). */
+  readonly form: 'notice'
+  /** One-line account of how the child ended. */
+  readonly summary: string
+  /** Session id of the child that settled. */
+  readonly senderSessionId: SessionId
+}
+```
+
 ```ts type-equiv
 /** Options for one continuable child's report to its direct parent. */
 interface SubagentReportOptions {
@@ -621,7 +642,7 @@ async start(name: string, request: SubagentStartRequest): Promise<SubagentRun>
 
 Types: [Agent](core.md) · [ContentBlock](llm-streaming.md) · [MessageId](llm-streaming.md) · [SessionId](core.md)
 
-Source: [`packages/subagent/subagent/src/index.ts:170`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:171`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagent-events"></a>
 
@@ -647,7 +668,7 @@ A published child settled. Scope-filtered dispatch uses the same delegating pare
 
 Types: [Scoped](scope.md)
 
-Source: [`packages/subagent/subagent/src/index.ts:165`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:166`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagentprovider-added--emit"></a>
 
@@ -664,7 +685,7 @@ A provider became resolvable in the registry.
 'subagent/provider-added'(provider: SubagentProvider): void
 ```
 
-Source: [`packages/subagent/subagent/src/index.ts:139`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:140`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagentprovider-removed--emit"></a>
 
@@ -681,7 +702,7 @@ A provider left the registry. Accepted runs remain holder-owned.
 'subagent/provider-removed'(name: string): void
 ```
 
-Source: [`packages/subagent/subagent/src/index.ts:145`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:146`](../../packages/subagent/subagent/src/index.ts)
 
 <a id="subagentstart--emit"></a>
 
@@ -705,5 +726,5 @@ A provider established a published child. For in-process providers, `ctx.agents.
 
 Types: [Scoped](scope.md)
 
-Source: [`packages/subagent/subagent/src/index.ts:156`](../../packages/subagent/subagent/src/index.ts)
+Source: [`packages/subagent/subagent/src/index.ts:157`](../../packages/subagent/subagent/src/index.ts)
 <!-- END GENERATED cordis-surface -->