docs: schedules live in root crontab, not /etc/cron.d or timers

Standardised 2026-08-31 on regular crontabs: nuc's push moved off its
systemd timer (units kept disabled on disk), and the cron.d files on
nuc and nas were folded into root's crontab. Notes the consequence
accepted for nuc: a night with the box powered off is skipped rather
than caught up after boot.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

nuc/usb4t-dropouts.md

@@ -10,7 +10,7 @@
 > host.
 
 Symptom seen first in the nightly backup log
-(`/var/log/incus-copy.log`, job in `/etc/cron.d/incus-copy`): every
+(`/var/log/incus-copy.log`, job in root's crontab): every
 instance fails with
 
 ```
@@ -140,7 +140,7 @@ before). Two reasons:
 
 Fixed 2026-08-30 by adding
 [`zpool-health.sh`](https://git.lutran.fr/julien/scripts/src/branch/main/zpool-health.sh),
-run every 15 min from `/etc/cron.d/zpool-health`. It mails only on
+run every 15 min from root's crontab. It mails only on
 `healthy <-> problem` **transitions**, so it is silent in normal
 operation and cannot spam; `-t` sends a test. Worth deploying on ks4
 too — its `data` pool is single-disk and has the same blind spot.