The curator's inactivity prune archived any non-pinned agent-created
skill whose activity was older than archive_after_days (90d). A skill
loaded only by a cron job had its usage bumped solely when the job
fired, so paused jobs, infrequent (quarterly/annual) schedules, and
far-future one-shots aged their skills out from under them — the next
run then failed to load the now-archived skill.
- cron/jobs.py: add referenced_skill_names() returning skills used by
ANY job (incl. paused/disabled).
- curator.apply_automatic_transitions(): skip cron-referenced skills
like pinned; add a use=0 grace floor so a never-used skill is not
marked stale/archived until it is at least stale_after_days old.
- LLM review pass: candidate list marks cron=yes; prompt forbids
pruning cron-referenced skills and never-used skills under 30 days.
Tested E2E against a real cron job + real usage records and with 4 new
unit tests.