Adds three pre-run gate recipes to the cron docs:
- file-change gate (stat + mtime + state file)
- external-flag gate (file presence)
- SQL-count gate (user's own database, not state.db)
These are the use cases @iankar8 proposed adding as a parallel
'trigger' subsystem in #2654. The existing `script` + `wakeAgent`
gate already covers all three at $0 — this lands the patterns as
documentation so users can find them, instead of adding a second
gating mechanism to the cron subsystem.
05d9f641c0