refactor: move host-crontab generation into cron plugin

Host-crontab generation for `app.json` cron tasks now lives in the `cron` plugin, gated by a new `scheduler-uses-host-cron` trigger that the `docker-local` scheduler answers true while self-managed schedulers such as `k3s` answer false. This lets any host-cron scheduler participate in normal `app.json` cron without coupling to `scheduler-docker-local` or duplicating the crontab writer, while the `k3s` scheduler continues to manage its own in-cluster cron jobs.

Closes #8862.

plugins/scheduler-k3s/src/triggers/triggers.go

@@ -86,6 +86,9 @@ func main() {
 		scheduler := flag.Arg(0)
 		appName := flag.Arg(1)
 		err = scheduler_k3s.TriggerSchedulerIsDeployed(scheduler, appName)
+	case "scheduler-uses-host-cron":
+		scheduler := flag.Arg(0)
+		err = scheduler_k3s.TriggerSchedulerUsesHostCron(scheduler)
 	case "scheduler-logs":
 		var tail bool
 		var quiet bool