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/src/fetch/client.ts

@@ -58,6 +58,7 @@ import {
 import { llmDiscoverModelsValueSchema, llmModelsValueSchema, llmProvidersValueSchema } from '../api/llm.schema.ts'
 import {
   subagentHistoryValueSchema,
+  subagentInterruptValueSchema,
   subagentListValueSchema,
   subagentPromptValueSchema,
 } from '../api/subagents.schema.ts'
@@ -96,6 +97,7 @@ export interface IApiClient {
     list(payload: RequestPayload<'subagent.list'>, signal?: AbortSignal): Promise<RpcResponse<ResponseValue<'subagent.list'>>>
     history(payload: RequestPayload<'subagent.history'>, signal?: AbortSignal): Promise<RpcResponse<ResponseValue<'subagent.history'>>>
     prompt(payload: RequestPayload<'subagent.prompt'>, signal?: AbortSignal): Promise<RpcResponse<ResponseValue<'subagent.prompt'>>>
+    interrupt(payload: RequestPayload<'subagent.interrupt'>, signal?: AbortSignal): Promise<RpcResponse<ResponseValue<'subagent.interrupt'>>>
   }
   host: {
     describe(payload: RequestPayload<'host.describe'>, signal?: AbortSignal): Promise<RpcResponse<ResponseValue<'host.describe'>>>
@@ -171,6 +173,7 @@ const UNARY_VALUE_SCHEMAS: { [K in keyof RpcMethodMap]: z.ZodType<Wire<ResponseV
   'subagent.list': subagentListValueSchema,
   'subagent.history': subagentHistoryValueSchema,
   'subagent.prompt': subagentPromptValueSchema,
+  'subagent.interrupt': subagentInterruptValueSchema,
   'host.describe': hostDescribeValueSchema,
   'host.pickDirectory': hostPickDirectoryValueSchema,
   'host.listDirectory': hostListDirectoryValueSchema,
@@ -407,6 +410,7 @@ export abstract class AbstractApiClient implements IApiClient {
     list: (payload, signal) => this.callUnary('subagent.list', payload, signal),
     history: (payload, signal) => this.callUnary('subagent.history', payload, signal),
     prompt: (payload, signal) => this.callUnary('subagent.prompt', payload, signal),
+    interrupt: (payload, signal) => this.callUnary('subagent.interrupt', payload, signal),
   }
 
   readonly host: IApiClient['host'] = {

packages/host/apiproxy/src/fetch/handler.ts

@@ -60,6 +60,7 @@ import {
 import { llmDiscoverModelsRequestSchema, llmModelsRequestSchema, llmProvidersRequestSchema } from '../api/llm.schema.ts'
 import {
   subagentHistoryRequestSchema,
+  subagentInterruptRequestSchema,
   subagentListRequestSchema,
   subagentPromptRequestSchema,
 } from '../api/subagents.schema.ts'
@@ -95,6 +96,7 @@ const UNARY_ROUTES: UnaryRoutes = {
   'subagent.list': { schema: subagentListRequestSchema, invoke: (api, r, signal) => api.subagents.list(r, signal) },
   'subagent.history': { schema: subagentHistoryRequestSchema, invoke: (api, r, signal) => api.subagents.history(r, signal) },
   'subagent.prompt': { schema: subagentPromptRequestSchema, invoke: (api, r, signal) => api.subagents.prompt(r, signal) },
+  'subagent.interrupt': { schema: subagentInterruptRequestSchema, invoke: (api, r) => api.subagents.interrupt(r) },
   'host.describe': { schema: hostDescribeRequestSchema, invoke: (api, r) => api.host.describe(r) },
   'host.pickDirectory': { schema: hostPickDirectoryRequestSchema, invoke: (api, r, signal) => api.host.pickDirectory(r, signal) },
   'host.listDirectory': { schema: hostListDirectoryRequestSchema, invoke: (api, r, signal) => api.host.listDirectory(r, signal) },