Config-version migrations have been observed to leave cron/jobs.json
valid-but-empty after `hermes update`, silently dropping every scheduled
job (#34600). The existing malformed-shape guards in cron/jobs.py don't
catch this because {"jobs": []} is valid JSON.
Add restore_cron_jobs_if_emptied() as a post-migration safety net: if the
live cron/jobs.json now has zero jobs while the pre-update snapshot held
one or more, restore the snapshot copy in place and warn loudly. The
check is conservative — it only restores on unambiguous evidence of loss
(snapshot had jobs, live file readable-and-empty), so a user who genuinely
cleared their jobs is never second-guessed and an unreadable live file is
left untouched so real corruption still surfaces.
Wired into _cmd_update_impl after migrate_config(), reusing the existing
pre-update quick snapshot (which already captures cron/jobs.json).
Closes#34600
