feat(subagent): add current-turn interrupt RPC

ctx.subagents.interrupt() stops one live continuable child's current turn
via Agent.cancel(cause, { keepInbox: true }) under either a human durable
parent address or an exact live ancestor Agent. Fire-and-return: admission
is synchronous, quiescence is not awaited. Pending inbox work, the
Activation, and published descendants are preserved; only a later waking
send resumes the parked FIFO queue. Absent, one-shot, and disposing
targets are accepted no-ops.

The new Host RPC subagent.interrupt calls only that primitive with user
authority — no catalog, history, persistence, or parent-registry lookup —
so a live child stays stoppable while its parent Agent is offline.

Refs #1535

packages/client/runtime/tests/fake-api.ts

@@ -140,10 +140,14 @@ export class FakeApiClient implements IApiClient {
   onSubagentPrompt: (payload: unknown) => Promise<RpcResponse<{ messageId: never }>>
     = () => Promise.resolve(ok({ messageId: 'fake-message' as never }))
 
+  onSubagentInterrupt: (payload: unknown) => Promise<RpcResponse<{ accepted: true }>>
+    = () => Promise.resolve(ok({ accepted: true as const }))
+
   readonly subagents: IApiClient['subagents'] = {
     list: (payload: unknown) => this.record('subagent.list', payload, this.onSubagentList(payload)),
     history: (payload: unknown) => this.record('subagent.history', payload, this.onSubagentHistory(payload)),
     prompt: (payload: unknown) => this.record('subagent.prompt', payload, this.onSubagentPrompt(payload)),
+    interrupt: (payload: unknown) => this.record('subagent.interrupt', payload, this.onSubagentInterrupt(payload)),
   }
 
   readonly host: IApiClient['host'] = {