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/config-catalog.md

@@ -1928,7 +1928,7 @@ export interface Config {
 
 Depends on: [`AgentOptions`](core-data-structures/core.md)
 
-Source: [`packages/subagent/tool-subagent/src/index.ts:24`](../packages/subagent/tool-subagent/src/index.ts)
+Source: [`packages/subagent/tool-subagent/src/index.ts:27`](../packages/subagent/tool-subagent/src/index.ts)
 
 ## `@deepseek-ai/dsh-tool-tasks`
 
@@ -2343,10 +2343,12 @@ These load from a `cordis.yml` entry with no `config:` block; they declare no co
 - `@deepseek-ai/dsh-session-projection` ([`packages/session-projection/session-projection/src/index.ts`](../packages/session-projection/session-projection/src/index.ts))
 - `@deepseek-ai/dsh-storage` ([`packages/storage/storage/src/index.ts`](../packages/storage/storage/src/index.ts))
 - `@deepseek-ai/dsh-subagent` ([`packages/subagent/subagent/src/index.ts`](../packages/subagent/subagent/src/index.ts))
+- `@deepseek-ai/dsh-subagent-control` — requires `subagents` · `tasks` · `agents` ([`packages/subagent/subagent-control/src/index.ts`](../packages/subagent/subagent-control/src/index.ts))
 - `@deepseek-ai/dsh-subprocess-local` ([`packages/subprocess/subprocess-local/src/index.ts`](../packages/subprocess/subprocess-local/src/index.ts))
 - `@deepseek-ai/dsh-tasks-local` ([`packages/tasks/tasks-local/src/index.ts`](../packages/tasks/tasks-local/src/index.ts))
 - `@deepseek-ai/dsh-timeout-policy` — requires `tools` ([`packages/timeout/timeout-policy/src/index.ts`](../packages/timeout/timeout-policy/src/index.ts))
 - `@deepseek-ai/dsh-tool-ask-user` — requires `tools` · `userInteraction` ([`packages/ui/tool-ask-user/src/index.ts`](../packages/ui/tool-ask-user/src/index.ts))
+- `@deepseek-ai/dsh-tool-subagent-control` — requires `tools` · `subagentControl` ([`packages/subagent/tool-subagent-control/src/index.ts`](../packages/subagent/tool-subagent-control/src/index.ts))
 - `@deepseek-ai/dsh-tool-todo` — requires `tools` ([`packages/todo/tool-todo/src/index.ts`](../packages/todo/tool-todo/src/index.ts))
 - `@deepseek-ai/dsh-typert-registry` ([`packages/typert/registry/src/index.ts`](../packages/typert/registry/src/index.ts))
 - `@deepseek-ai/dsh-user-interaction` ([`packages/ui/user-interaction/src/index.ts`](../packages/ui/user-interaction/src/index.ts))