Function spawn_health_watchdog 

pub(crate) fn spawn_health_watchdog(health_monitor: &Arc<HealthMonitor>)

Spawn the health-watchdog background task for a turn.

The watchdog polls health_monitor.check() every 5 s while a turn is active and exits as soon as is_active_turn() returns false (i.e. after the turn ends or the process exits). Duplicated verbatim in persistent.rs and subprocess.rs (twice) before this extraction.