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/architecture.zh.md

@@ -39,6 +39,7 @@
 | `ctx.web` | [`web/`](../packages/web/README.md) | 搜索与抓取提供方注册表 |
 | `ctx.compact`，`ctx.toolResultPrune` | [`compact/`](../packages/compact/README.md)/[`compact-tool-result-prune`](../packages/compact/compact-tool-result-prune/README.md) | 摘要压缩（compaction）和可选的无模型结果裁剪 |
 | `ctx.subagents` | [`subagent/`](../packages/subagent/README.md) | 具名委托提供方 |
+| `ctx.subagentControl` | [`subagent/`](../packages/subagent/README.md) | 可继续子 agent 的 Task 化 activation，以及 steer 或恢复路由 |
 | `ctx.planMode` | [`plan/`](../packages/plan/README.md) | 落日志的 plan 协作状态 |
 | `ctx.tasks` | [`tasks/`](../packages/tasks/README.md) | 后台任务注册表和通用 `task_*` 控制 |
 | `ctx.workflows` | [`workflow/`](../packages/workflow/README.md) | 脚本驱动的多 agent 编排 |