fix(delegation): add hard timeout and stale detection for subagent execution (#13770)

- Wrap child.run_conversation() in a ThreadPoolExecutor with configurable
  timeout (delegation.child_timeout_seconds, default 300s) to prevent
  indefinite blocking when a subagent's API call or tool HTTP request hangs.

- Add heartbeat stale detection: if a child's api_call_count doesn't
  advance for 5 consecutive heartbeat cycles (~2.5 min), stop touching
  the parent's activity timestamp so the gateway inactivity timeout
  can fire as a last resort.

- Add 'timeout' as a new exit_reason/status alongside the existing
  completed/max_iterations/interrupted states.

- Use shutdown(wait=False) on the timeout executor to avoid the
  ThreadPoolExecutor.__exit__ deadlock when a child is stuck on
  blocking I/O.

Closes #13768

scripts/release.py

@@ -334,6 +334,7 @@ AUTHOR_MAP = {
     "asslaenn5@gmail.com": "Aslaaen",
     "shalompmc0505@naver.com": "pinion05",
     "105142614+VTRiot@users.noreply.github.com": "VTRiot",
+    "vivien000812@gmail.com": "iamagenius00",
 }