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.

scripts/gen-doc-graphs.ts

@@ -430,6 +430,14 @@ const SERVICE_ROLES: ServiceRole[] = [
     consumers: ['tool-subagent', 'tool-ralph'],
     note: 'Providers implement transports; tool-subagent exposes configured delegation while tool-ralph requires one fresh structured-output route.',
   },
+  {
+    key: 'subagentControl',
+    pkg: 'subagent',
+    title: 'Continuable-subagent control service',
+    mode: 'core',
+    consumers: ['tool-subagent', 'tool-subagent-control'],
+    note: 'Binds one durable child session to Task-backed activations over ctx.subagents; tool-subagent starts continuable background children and tool-subagent-control delivers follow-up messages.',
+  },
   {
     key: 'tasks',
     pkg: 'tasks',