Use a fast `defaults read` pre-check before spawning any tmutil
process. On machines without Time Machine configured the check
returns instantly, avoiding the spinner and the 2-3s tmutil timeout.
Affected locations:
- lib/clean/system.sh: clean_time_machine_failed_backups()
- lib/clean/system.sh: clean_local_snapshots()
- lib/clean/user.sh: local snapshot hint in system hints