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-tool-catalog.ts

@@ -28,6 +28,8 @@ import * as WebSearchExa from '@deepseek-ai/dsh-web-search-exa'
 import * as WebFetchLocal from '@deepseek-ai/dsh-web-fetch-local'
 import SubagentService from '@deepseek-ai/dsh-subagent'
 import type { SubagentProvider } from '@deepseek-ai/dsh-subagent'
+import SubagentControlService from '@deepseek-ai/dsh-subagent-control'
+import * as ToolSubagentControl from '@deepseek-ai/dsh-tool-subagent-control'
 import SkillService from '@deepseek-ai/dsh-skill'
 import * as SkillLocal from '@deepseek-ai/dsh-skill-local'
 import LocalTaskService from '@deepseek-ai/dsh-tasks-local'
@@ -106,6 +108,9 @@ function registerCatalogSubagentProvider(ctx: Context, name: string): void {
     capabilities: { outputSchema: true, depthLimit: true, toolFilter: true, persona: true },
     inheritsParentContext: false,
     start: () => Promise.reject(new Error('tool-catalog provider cannot start a child')),
+    // Presence marks the continuation capability, so tool-subagent harvests
+    // its shipped continuable background wording (spawn/fork are resumable).
+    resume: () => Promise.reject(new Error('tool-catalog provider cannot resume a child')),
   }
   ctx.subagents.registerProvider(provider)
 }
@@ -379,6 +384,22 @@ const TOOL_PACKAGES: ToolPackage[] = [
     note:
       'The registered tool name is the load-time `toolName` config (default `subagent`); the schema above is that default. The shipped example agents load this package once per subagent backend, so the model additionally sees `subagent_fork` (bound to the fork backend) with an identical schema — see `apps/cli/config/base.cordis.yml` and `examples/acp-agent/cordis.yml`.',
   },
+  {
+    pkg: '@deepseek-ai/dsh-tool-subagent-control',
+    dir: 'tool-subagent-control',
+    source: 'packages/subagent/tool-subagent-control/src/index.ts',
+    requires: ['ctx.tools', 'ctx.subagentControl'],
+    writes: ['tool/call', 'tool/result', 'child session events through the control service'],
+    async mount(ctx) {
+      await ctx.plugin(SubagentService)
+      await ctx.plugin(LocalTaskService)
+      await ctx.plugin(AgentRegistry)
+      await ctx.plugin(SubagentControlService)
+      await ctx.plugin(ToolSubagentControl)
+    },
+    note:
+      'The one globally named follow-up tool over continuable background subagents: provider-bound `tool-subagent` instances register distinct delegation tools, while this package registers `send_message` once.',
+  },
   {
     pkg: '@deepseek-ai/dsh-tool-tasks',
     dir: 'tool-tasks',