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/src/index.ts

@@ -288,8 +288,10 @@ export function apply(ctx: Context, config: Config): void {
             // The schema above tells the model to follow up with
             // `send_message`; starting a durable child the model cannot
             // continue would make that advertisement false. Sibling load order
-            // is undetermined at mount, so the check lives at the operation.
-            if (ctx.tools.get('send_message') === undefined) {
+            // is undetermined at mount, so the check lives at the operation,
+            // and it resolves in the CALLER's scope so a restriction that
+            // removes send_message from this agent also blocks the start.
+            if (ctx.tools.get('send_message', parent) === undefined) {
               throw new Error('continuable background subagents unavailable: load @deepseek-ai/dsh-tool-subagent-control (the advertised send_message tool is not registered)')
             }
             // The control service owns the durable child id, descriptor