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/subagent/tool-subagent/tests/tool-subagent.spec.ts

@@ -903,6 +903,22 @@ describe('dsh-tool-subagent continuable background mode', () => {
     // Nothing was started: no Task exists for the parent.
     expect(ctx.tasks.list(parent)).toEqual([])
   })
+
+  it('resolves send_message availability in the CALLER scope, not the global registry', async () => {
+    // A scoped restriction that keeps this delegation tool but removes
+    // send_message means this agent cannot execute the promised follow-up;
+    // the availability check must see the caller's surface.
+    const { ctx, parent } = await continuableSetup()
+    parent.ctx.tools.restrict({ deny: ['send_message'] })
+    const result = await callSubagent(
+      ctx,
+      { description: 'd', prompt: 'p', run_in_background: true },
+      { agent: parent },
+    )
+    expect(result.isError).toBe(true)
+    expect(text(result)).toContain('load @deepseek-ai/dsh-tool-subagent-control')
+    expect(ctx.tasks.list(parent)).toEqual([])
+  })
 })
 
 describe('background preflight failure (no orphaned child, by construction)', () => {