feat(subagent): continuable background subagents

Implement the continuable background subagents RFC: a durable child
session with a series of Task-backed activations, each disposing its
run before the Task settles.

- dsh-subagent: rename SubagentRun.sendMessage to strict steer, drop
  run-level resume, add SubagentProvider.resume dispatch via
  SubagentService.resume, the continuation start field, and the
  versioned model-hidden subagent/descriptor session event.
- dsh-subagent-inprocess/-spawn/-fork: publish the control-allocated
  child id, append the descriptor inside the initial turn, implement
  cold resume from the child's own transcript under the live parent
  scope, and strict running-only steer.
- dsh-subagent-control (new): SubagentControlService owning stable
  child ids, descriptor snapshot/fold/authorization, Task-backed
  activation with settle-then-dispose ordering, the process-local
  active-run association, and steer-or-resume sendMessage routing.
- dsh-tool-subagent: background route branches on the provider's
  resume capability (continuable via the control service; one-shot
  task for ACP), returning both child and task ids.
- dsh-tool-subagent-control (new): the globally named send_message
  tool rendering steered/started routes.

Keyless coverage spans Task ownership and disposal ordering, running
delivery, cold follow-up, descriptor rejection and rollback, known-id
reconstruction, kill during lookup, admission races, and a new
subagent-continuable ACP snapshot scenario.

packages/subagent/subagent-spawn/tests/subagent-spawn.spec.ts

@@ -235,12 +235,19 @@ describe('dsh-subagent-spawn', () => {
     expect(result.stopReason).toBe('aborted')
   })
 
-  it('does not expose the optional runtime methods (sendMessage/resume) in this cut', async () => {
+  it('exposes strict steer (no run-level resume): a settled child throws instead of queueing', async () => {
     const { ctx, parent } = await setup([textResponse('x')])
     const run = await start(ctx, 'spawn', { prompt: [{ type: 'text', text: 'p' }], parent })
-    expect('sendMessage' in run).toBe(false)
+    // A run represents one disposable activation: cold resume is a provider
+    // method, never a run method.
     expect('resume' in run).toBe(false)
+    expect(typeof run.steer).toBe('function')
     await run.result
+    // Strict live-only contract: after the child settles, delivery fails loud
+    // rather than falling back to Agent.steer()'s idle queue (which would
+    // start an untracked turn).
+    expect(() => { run.steer!([{ type: 'text', text: 'late' }]) })
+      .toThrow(/not running; the message was not delivered/)
     await run.dispose()
   })