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/tool-subagent/README.md

@@ -10,7 +10,7 @@ Each plugin instance binds one `provider` to one `toolName`; the model receives
 
 A foreground call passes the execution signal through startup and execution, awaits `run.result`, and always awaits `run.dispose()` before returning. Only `completed` returns the canonical `{ kind: 'foreground', runId, output: JsonValue[] }`, rendered as the same final text; abort, refusal, token limit, and other failures become errored tool results without partial output.
 
-With `run_in_background: true`, the tool registers the parent-owned task before starting the provider and returns canonical `{ kind: 'background', taskId }`, rendered as `started background subagent task <id>`. A task-owned signal covers pending startup and the child after the starting call returns. `task_kill` and owner disposal abort it. Settlement awaits startup rollback or child disposal, then maps completed final text, abort to `killed`, and other failures to `failed`. The task has no incremental read; generic task tools own later status, collection, cancellation, and notices. See the [background subagent Agent Note](../../../.agents/notes/implemented/feature/2026-07-08-background-subagent-tasks.md).
+With `run_in_background: true`, the route follows the provider's continuation capability and returns canonical `{ kind: 'background', taskId, subagentId? }`. A resumable provider (spawn, fork) delegates to `ctx.subagentControl.startContinuable()`, which owns the durable child id, descriptor snapshot, Task registration, and settle-then-dispose ordering; the result includes `subagentId`, renders as `started subagent <childId> as task <taskId>`, and accepts follow-up messages through the global `send_message` tool. A one-shot provider (ACP) keeps the plain parent-owned task, omits `subagentId`, and renders as `started background subagent task <id>`. Either way a task-owned signal covers pending startup and the child after the starting call returns; `task_kill` and owner disposal abort it, settlement awaits startup rollback or child disposal, and completed final text, abort to `killed`, and other failures to `failed` map identically. The task has no incremental read; generic task tools own later status, collection, cancellation, and notices. See the [background subagent Agent Note](../../../.agents/notes/implemented/feature/2026-07-08-background-subagent-tasks.md) and the [continuable background subagents Agent Note](../../../.agents/notes/implemented/feature/2026-07-21-continuable-background-subagents.md).
 
 `toolFilter` changes the child's global tool layer but is not a parent-derived authority ceiling. See the [agent-scope security non-goal](../../../.agents/notes/implemented/architecture/2026-07-08-agent-scope-contexts.md#security-and-authority-are-non-goals).
 
@@ -64,7 +64,7 @@ Append-only; newly visible content follows the reusable request prefix and does
 
 #### What the model sees
 
-Start returns exactly `started background subagent task <id>`. The generic task surface provides later status, final output, cancellation responses, and notices.
+Start returns exactly `started subagent <childId> as task <taskId>` on a resumable provider, or `started background subagent task <id>` on a one-shot provider. The generic task surface provides later status, final output, cancellation responses, and notices; `send_message` (from `dsh-tool-subagent-control`) delivers follow-ups to a continuable child.
 
 #### Token effect