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/connection/src/client/fixture.ts

@@ -2203,6 +2203,7 @@ function createFixtureWorld(options: FixtureOptions): FixtureWorld {
       prompt: request => Promise.resolve(ok(request, {
         messageId: `fixture-message-${request.payload.childSessionId}` as never,
       })),
+      interrupt: request => Promise.resolve(ok(request, { accepted: true as const })),
     },
     host: {
       describe: request => ok(request, { version: '0.0.0-fixture', cwd: '/tmp/fixture', attachedSessions }),
@@ -2748,6 +2749,7 @@ export class FixtureApiClient extends AbstractApiClient {
       case 'subagent.list': return this.api.subagents.list(request)
       case 'subagent.history': return this.api.subagents.history(request)
       case 'subagent.prompt': return this.api.subagents.prompt(request, signal)
+      case 'subagent.interrupt': return this.api.subagents.interrupt(request)
       case 'host.describe': return this.api.host.describe(request)
       case 'host.pickDirectory': return this.api.host.pickDirectory(request, new AbortController().signal)
       case 'host.listDirectory': return this.api.host.listDirectory(request, new AbortController().signal)