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/host/apiproxy/tests/client-handler.spec.ts

@@ -63,6 +63,7 @@ function scriptedApi(overrides: {
       list: r => ok(r, { entries: [], parentAvailable: false }),
       history: r => ok(r, { events: [], hasMore: false }),
       prompt: r => ok(r, { messageId: 'message-1' as never }),
+      interrupt: r => ok(r, { accepted: true as const }),
       ...overrides.subagents,
     },
     host: {
@@ -248,6 +249,32 @@ describe('unary round trip', () => {
     }
   })
 
+  it('round-trips subagent.interrupt and rejects a one-shot or incomplete address', async () => {
+    const interrupt = vi.fn((r: RpcRequest<unknown>) => ok(r, { accepted: true as const }))
+    const api = scriptedApi({ subagents: { interrupt } })
+    const c = client(api)
+
+    const accepted = await c.subagents.interrupt({
+      parentSessionId: sid('parent'), childSessionId: sid('child'), mode: 'continuable',
+    })
+    expect(accepted.result).toEqual({ ok: true, value: { accepted: true } })
+    expect(interrupt).toHaveBeenCalledTimes(1)
+
+    // The wire schema owns the mode fence: a one-shot address never reaches the impl.
+    const oneShot = await c.subagents.interrupt({
+      parentSessionId: sid('parent'), childSessionId: sid('child'), mode: 'one-shot',
+    } as never)
+    expect(oneShot.result.ok).toBe(false)
+    if (!oneShot.result.ok) expect(oneShot.result.error.code).toBe('bad-request')
+
+    const incomplete = await c.subagents.interrupt({
+      parentSessionId: sid('parent'), mode: 'continuable',
+    } as never)
+    expect(incomplete.result.ok).toBe(false)
+    if (!incomplete.result.ok) expect(incomplete.result.error.code).toBe('bad-request')
+    expect(interrupt).toHaveBeenCalledTimes(1)
+  })
+
   it('rejects a method/path mismatch as bad-request', async () => {
     const handler = toFetchHandler(scriptedApi())
     const body = { type: 'client-request', rpcId: 'r1', method: 'session.create', payload: {} }