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.

docs/persistence-catalog.md

@@ -516,6 +516,23 @@ Source: [`packages/core/session/src/types.ts:218`](../packages/core/session/src/
 
 Source: [`packages/core/session/src/types.ts:216`](../packages/core/session/src/types.ts)
 
+### `subagent/*`
+
+#### `subagent/descriptor` — log-only
+
+```ts persistence-catalog
+/**
+ * Durable declared composition of a continuable subagent child, appended
+ * once by the establishing provider inside the child's initial turn,
+ * before its first request. Log-only: it carries no `surfaceOp`, never
+ * enters model history, and the append-only log retains it when
+ * compaction replaces surface history.
+ */
+'subagent/descriptor': SubagentDescriptorData
+```
+
+Source: [`packages/subagent/subagent/src/descriptor.ts:32`](../packages/subagent/subagent/src/descriptor.ts)
+
 ### `todo/*`
 
 #### `todo/write` — log-only