docs(subagent): record the public interrupt in the continuation contract docs

The continuable-subagent core-data page and the continuation lifecycle
Agent Note claimed no public subagent cancellation existed; interrupt()
is now that one public stop. Both pages and their Chinese counterparts
state the shipped contract and link the owning interrupt note.

Refs #1535

docs/core-data-structures/subagent.md

@@ -139,7 +139,20 @@ The Agent inbox is the only queue. Every continuation message becomes one `Agent
 
 Follow-up authority comes from an exact live Agent tool context. The authenticated Agent must be the durable child's direct parent recorded in `SessionHeader.parentSession`. `MessageSource` and `senderSessionId` are durable provenance after admission and grant no authority; the optional model-facing tool uses `CoordinatorMessageSource`.
 
-For both operations the caller signal owns lookup, materialization, and admission only until inbox acceptance. Afterwards the manager owns the Activation independently: later caller cancellation neither cancels the accepted turn nor disposes the child, and the seam exposes no public subagent cancellation or steering operation.
+For both operations the caller signal owns lookup, materialization, and admission only until inbox acceptance. Afterwards the manager owns the Activation independently: later caller cancellation neither cancels the accepted turn nor disposes the child, and the seam exposes no steering operation.
+
+`SubagentService.interrupt(targetSessionId, authority)` is the one public stop: it authorizes synchronously, issues `Agent.cancel(cause, { keepInbox: true })` on the live target, and returns without awaiting quiescence. The Activation, its pending inbox work, and published descendants are untouched; only a later waking send resumes the parked FIFO queue. An absent target — unknown, one-shot, or already settled — and a manager-less composition are accepted no-ops; a wrong parent address or a stale, self-targeting, or non-ancestor caller rejects with `UNAUTHORIZED`.
+
+```ts type-equiv
+/**
+ * Authority under which one interrupt request is admitted. `user` carries the
+ * durable direct-parent address a human client presented; `ancestor` carries
+ * the exact live Agent object whose recorded lineage must contain the caller.
+ */
+type SubagentInterruptAuthority =
+  | { readonly kind: 'user'; readonly parentSessionId: SessionId }
+  | { readonly kind: 'ancestor'; readonly agent: Agent }
+```
 
 Every Activation owns its `AgentHandle` and an `ownedChildren: Set<SessionId>`; because one Session has at most one live Activation, the child Session id identifies the live child without another runtime-incarnation reference. Starting a child or submitting parent-originated work registers the child in a continuation-managed parent's set before the child can run, and that parent cannot settle while the set is non-empty. A top-level or other non-continuation Agent has no Activation and stays outside the waiting graph. Child release happens only after the child Agent is quiescent, every child of that child is disposed, the best-effort final session flush settles, and the child's `AgentHandle` completes disposal.