Create varaverk.plg on flash (/boot/config/plugins/varaverk/varaverk.plg).
On boot, rebuild_cron copies it to /var/log/plugins/ so update_cron natively
picks up varaverk.cron and merges it into /etc/cron.d/root.
With the .plg in place the dev-mode direct-inject fallback in vv_cron_rebuild()
is no longer needed and has been removed.
/etc/cron.d/varaverk is RAM-based and wiped on every reboot. The
disks_mounted event fires early in boot (before Docker) with /boot
already readable, so schedule.json is available to restore the cron
file automatically without any user action.