fix(sdk): mount task controls for subagents

.agents/notes/implemented/feature/2026-07-21-continuable-background-subagents.md

@@ -35,7 +35,7 @@ Every later turn creates another Task. Its producer resources cover only that ac
 
 Opening a child session in a human-facing adapter reads its persisted transcript and does not resume an Agent merely to display it. Human input starts or joins the same Task-backed activation used by parent input through the control service. A human-started Task retains the exact currently loaded parent Agent as its notification target, and `task_output` remains the single result path. The existing completion listener injects at most one unsolicited notice while the Task is unreported; `kill`, a terminal read, or a terminal wait may mark it reported and suppress that notice. Human interaction is therefore permitted only while that parent instance remains live. A user-owned conversation that may outlive the parent and explicitly merge a conclusion back belongs to [interactive side sessions](../../proposed/feature/2026-07-08-interactive-side-sessions.md), not this Task-owned lifecycle.
 
-`TaskService.start()` rejects producers when no Task control surface is attached. A human-facing adapter that accepts child input must therefore attach a Task control surface, or run in a deployment that loads `@deepseek-ai/dsh-tool-tasks`; loading the Task service alone is insufficient. This dependency is the cost of using the same Task result, cancellation, and notification path for parent- and human-started activations.
+`TaskService.start()` rejects producers when no Task control surface is attached. A human-facing adapter that accepts child input must therefore attach a Task control surface, or run in a deployment that loads `@deepseek-ai/dsh-tool-tasks`; loading the Task service alone is insufficient. SDK-generated spawn and fork compositions mount `@deepseek-ai/dsh-tasks` and `@deepseek-ai/dsh-tool-tasks` with the subagent control pair. This dependency is the cost of using the same Task result, cancellation, and notification path for parent- and human-started activations.
 
 Cancellation always targets the whole current activation. If human and parent messages have joined one turn, either caller's cancellation aborts that turn, disposes its run, and settles its Task as `killed`; the messages do not have independent results or cancellation rights. Independent cancellation requires a later message to start a separate turn instead of steering the current one.
 
@@ -110,6 +110,7 @@ Task records and active-run associations are process-local. Persistence makes th
 - `packages/subagent/subagent-inprocess/tests/subagent-inprocess.spec.ts` pins the continuable durability boundary: a permanent flush failure rejects with `DURABILITY_FAILED` and its cause, a transient loop-checkpoint failure can succeed on the final confirmation, cancellation owns either final-checkpoint outcome, resume also confirms durability, and foreground runs remain best-effort. `packages/subagent/subagent-control/tests/subagent-control.spec.ts` drives the real stack (agent loop, JSONL persistence, spawn/fork providers, Task service and surface, control service) keylessly: initial and resumed activations create fresh Tasks and dispose their runs before terminal; the descriptor event is turn-enclosed, model-hidden, versioned, and durable under the control-allocated child id; `task_kill` during a run, a final durability checkpoint, or cold-resume lookup settles `killed` after quiescence with no child work; steering joins the running Task without a second Task and retains the caller source; cold follow-ups accumulate turns in one durable transcript with their source and declared composition reconstructed; fork resume keeps the persisted seed boundary and never re-forks newer parent history; resumed depth uses the persisted header floor; foreign-parent, descriptor-less, and unmaterialized ids fail their started Task with the id unavailable; ownership conflicts and steering-settlement races report not-delivered without cold-resume fallthrough; competing sends during resume load are admitted once.
 - `packages/subagent/tool-subagent-control/tests/tool-subagent-control.spec.ts` pins the `send_message` schema, coordinator attribution, both route renderings, the not-delivered failure, the no-agent rejection, and HMR disposal.
 - `packages/subagent/tool-subagent/tests/tool-subagent.spec.ts` covers the capability-branched background route: a resumable provider returns both ids through the control service and advertises `send_message`, a one-shot provider keeps the plain task acknowledgement, and a resumable provider without the control service fails loud.
+- `packages/sdk/helper/tests/project.spec.ts` pins the Task service and model-facing Task controls in generated spawn and fork compositions.
 - The keyless ACP snapshot scenario `subagent-continuable` (examples/acp-agent) pins the model-visible transcript: the two-id acknowledgement, a final durability-confirmation failure rendered through `task_output` without unconfirmed child output, and a `send_message` follow-up whose started Task fails with the id unavailable.
 
 ## Consequences