fix: Reduce parent-death watchdog CPU usage (#12697)
## Summary
- Remove the per-task target-exit monitor from Unix parent-death
watchdogs.
- Keep each watchdog blocked only on its control pipe so persistent
tasks do not create high-CPU kqueue/poll loops on macOS.
- Preserve parent-death cleanup: if Turbo exits without disarming the
watchdog, the captured process group is still terminated.
Closes #12693
