fix: address codex review round 2

- Wire the control service and send_message tool into every shipped
  composition with a resumable provider and background enabled
  (headless-agent, tui-agent, and the SDK helper's subagent feature
  base resources); jsonrpc-agent disables background and is unchanged.
- Resolve the send_message availability check in the CALLER's tool
  scope so a restriction that removes the follow-up tool from one
  agent also blocks that agent's continuable start.
- Control-service disposal now cancels live activations and awaits
  producer settlement instead of stranding them: TaskService keeps
  producer Tasks across a reload, so the disposing service aborts each
  activation-owned controller, resolves its terminal gate (the
  effect-scoped onTaskDone listener is already gone), and awaits done.
  A new test kills a mid-start activation through HMR disposal.

packages/sdk/helper/src/features/builtin/index.ts

@@ -209,7 +209,13 @@ config:
       id: 'subagent',
       summary: 'Delegate work to child agents',
       mode: 'multiple',
-      baseResources: [{ kind: 'npm-cordis-config-entry', id: 'subagent', package: '@deepseek-ai/dsh-subagent' }],
+      // The control pair rides every resumable in-process option: background
+      // delegation on spawn/fork is continuable and advertises send_message.
+      baseResources: [
+        { kind: 'npm-cordis-config-entry', id: 'subagent', package: '@deepseek-ai/dsh-subagent' },
+        { kind: 'npm-cordis-config-entry', id: 'subagent-control', package: '@deepseek-ai/dsh-subagent-control' },
+        { kind: 'npm-cordis-config-entry', id: 'tool-subagent-control', package: '@deepseek-ai/dsh-tool-subagent-control' },
+      ],
       options: [
         {
           id: 'spawn',