PLAN: schedules are cron jobs, and zfSnap is retired

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

PLAN.md

@@ -296,8 +296,24 @@ something long in one, switch away and back, refresh — it's all still there.
   **Nothing is destroyed unseen.** Every schedule answers *what would it delete?* with the
   list, sizes and dates, before Run now touches anything; only names that parse as a stamp
   and a TTL are candidates, so a snapshot taken by hand is invisible to it; and a snapshot
-  deleted by hand needs its full name typed back. The scheduler is a 30 s tick in the daemon,
-  and `Run now` calls the same code, so the button and the timer cannot drift apart.
+  deleted by hand needs its full name typed back.
+  **A schedule is a real cron job** (2026-09-24, Mitchell's call after the first cut ran it
+  from a ticker inside the daemon). Ham Lab writes `/etc/cron.d/hamlab-zfs`, owns it, and
+  rewrites it whole on every change; each line calls `hamlab zfs-snapshot <id>` as the
+  account Ham Lab runs as, reaching zfs through sudo as the daemon does. Snapshots are
+  backup-shaped and must not stop because a console is down, upgrading, or uninstalled — and
+  a cron line is greppable by anyone looking at the machine the normal way. **A cron
+  expression is the single description of when a schedule fires**: the four common shapes
+  render to one, `custom` states one outright, and the next run is read back with the same
+  evaluator the Jobs tab uses on everyone else's lines, so the page, the file and the
+  prediction cannot disagree. A run's outcome is written to `zfs-runs/<id>.json` beside the
+  config, never to `state.json`, because the cron job is a separate process and the daemon
+  would clobber it. An existing external job can be **adopted** — its schedule, retention
+  and dataset read out of the cron line into a new Ham Lab schedule.
+  **zfSnap is retired on `grid`** as of that day: Ham Lab now takes the nightly `tank`
+  snapshot, the package is purged and the old cron line is gone, with the 515 snapshots it
+  had taken left untouched and fully readable here — which is what adopting its naming
+  bought. See `hamlet/docs/hosts.md`.
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