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/tool-catalog.md

@@ -31,6 +31,7 @@ This table connects model-visible tool names to the plugin package and service s
 | `@deepseek-ai/dsh-tool-skill` | `skill` | `ctx.tools`, `ctx.agents`, `ctx.skills` | `tool/call`, `tool/result`, `user/message replacement catalogs via agent.inject()` | - | - |
 | `@deepseek-ai/dsh-tool-session-query` | `session_event_read`, `session_event_search`, `session_event_trace`, `session_search`, `session_trace` | `ctx.tools`, `ctx.systemPrompt`, `ctx.sessionQuery`, `a calling Agent for workspace authority` | `tool/call`, `tool/result` | - | The five read-only tools hide provider cursors and authorize every result from the immutable calling agent session. The package is opt-in; compositions that need enforced deadlines or bounded inline output also mount the generic timeout or spill policies. |
 | `@deepseek-ai/dsh-tool-subagent` | `subagent` | `ctx.tools`, `ctx.subagents` | `tool/call`, `tool/result`, `child session events through the chosen provider` | `subagent`, `subagent_fork` | 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`. |
+| `@deepseek-ai/dsh-tool-subagent-control` | `send_message` | `ctx.tools`, `ctx.subagentControl` | `tool/call`, `tool/result`, `child session events through the control service` | - | 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. |
 | `@deepseek-ai/dsh-tool-tasks` | `task_kill`, `task_list`, `task_output` | `ctx.tools`, `ctx.tasks`, `ctx.systemPrompt` | `tool/call`, `tool/result`, `user/message via agent.inject() for background completion notices` | - | The kind-agnostic background-task control surface: background bash commands, PTY sends, and subagents are read, listed, and killed through the same three tools. Loading the plugin attaches the control surface that arms producers' `ctx.tasks.start()`. |
 | `@deepseek-ai/dsh-tool-todo` | `todo_write` | `ctx.tools`, `owning Agent session` | `tool/call`, `todo/write`, `tool/result` | - | todo_write is session-owned state; UIs render the latest todo/write event as a checklist. |
 | `@deepseek-ai/dsh-tool-workflow` | `workflow` | `ctx.tools`, `ctx.workflows`, `ctx.systemPrompt`, `a calling Agent (exec.agent parents the script children)` | `tool/call`, `tool/result` | - | - |
@@ -1116,7 +1117,7 @@ The five read-only tools hide provider cursors and authorize every result from t
 
 ### `subagent`
 
-Delegate a self-contained task to a subagent (a separate agent that works in its own context) and return its final result. Use this to offload focused, independent work — research, a scoped implementation, an analysis — so it does not consume this conversation's context. The subagent runs to completion and you receive only its final answer, not its intermediate steps. Give it a complete, standalone prompt: it does not see this conversation. Set `run_in_background: true` to return a task id; collect with `task_output` and stop with `task_kill`.
+Delegate a self-contained task to a subagent (a separate agent that works in its own context) and return its final result. Use this to offload focused, independent work — research, a scoped implementation, an analysis — so it does not consume this conversation's context. The subagent runs to completion and you receive only its final answer, not its intermediate steps. Give it a complete, standalone prompt: it does not see this conversation. Set `run_in_background: true` to start a continuable background subagent: you receive its subagent id and a task id; collect the result with `task_output`, stop it with `task_kill`, and send follow-up messages with `send_message`.
 
 ```json
 {
@@ -1132,7 +1133,7 @@ Delegate a self-contained task to a subagent (a separate agent that works in its
     },
     "run_in_background": {
       "type": "boolean",
-      "description": "Run as a background task and return its id; collect with task_output or stop with task_kill."
+      "description": "Run as a continuable background subagent and return its subagent and task ids; collect with task_output, stop with task_kill, follow up with send_message."
     }
   },
   "required": [
@@ -1146,6 +1147,36 @@ Source: [`packages/subagent/tool-subagent/src/index.ts`](../packages/subagent/to
 
 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`.
 
+## `@deepseek-ai/dsh-tool-subagent-control`
+
+### `send_message`
+
+Send a follow-up message to a background subagent by its subagent id. If it is still working, the message joins its current task; if it has finished, this starts a new task that continues the same subagent conversation. Either way the response arrives through the returned task id — collect it with `task_output`. A failure means the message was NOT delivered.
+
+```json
+{
+  "type": "object",
+  "properties": {
+    "subagent_id": {
+      "type": "string",
+      "description": "The subagent id returned when the background subagent was started."
+    },
+    "message": {
+      "type": "string",
+      "description": "The message to deliver to the subagent."
+    }
+  },
+  "required": [
+    "subagent_id",
+    "message"
+  ]
+}
+```
+
+Source: [`packages/subagent/tool-subagent-control/src/index.ts`](../packages/subagent/tool-subagent-control/src/index.ts)
+
+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.
+
 ## `@deepseek-ai/dsh-tool-tasks`
 
 ### `task_kill`