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.

examples/acp-agent/tests/acp.snapshot.ts

@@ -213,6 +213,11 @@ const SCENARIOS: Scenario[] = [
   { name: 'subagent-multi', hasModelTurn: true, recorded: true },
   { name: 'subagent-fork', hasModelTurn: true, recorded: true },
   { name: 'subagent-mixed', hasModelTurn: true, recorded: true },
+  // Authored continuable-subagent transcript: a background delegation returns
+  // both the durable subagent id and its task id, task_output collects the
+  // child result after settlement, and send_message to an unknown subagent id
+  // starts a follow-up task that settles failed with the id unavailable.
+  { name: 'subagent-continuable', hasModelTurn: true, recorded: false },
   {
     name: 'subagent-depth-two-rejection',
     hasModelTurn: true,