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/api/index.ts

@@ -42,7 +42,8 @@ export type {
 } from './sessions.ts'
 export type { DirectoryEntry, DirectoryListing, HostApi } from './host.ts'
 export type {
-  SubagentAddress, SubagentCatalog, SubagentListEntry, SubagentPromptReceipt, SubagentsApi,
+  SubagentAddress, SubagentCatalog, SubagentInterruptReceipt, SubagentListEntry,
+  SubagentPromptReceipt, SubagentsApi,
 } from './subagents.ts'
 export type { WorkspaceApi, WorkspaceId, WorkspaceView } from './workspace.ts'
 export type { CommandsApi, CommandDescriptor } from './commands.ts'