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/Makefile

@@ -1,5 +1,5 @@
 SUBCOMMANDS = subcommands/annotations:set subcommands/annotations:report subcommands/autoscaling-auth:set subcommands/autoscaling-auth:report subcommands/charts:report subcommands/charts:set subcommands/cluster:add subcommands/cluster:list subcommands/cluster:remove subcommands/ensure-charts subcommands/initialize subcommands/labels:set subcommands/labels:report subcommands/preview subcommands/profiles:add subcommands/profiles:list subcommands/profiles:remove subcommands/report subcommands/set subcommands/show-kubeconfig subcommands/uninstall
-TRIGGERS = triggers/core-post-deploy triggers/core-post-extract triggers/install triggers/post-app-clone-setup triggers/post-app-rename-setup triggers/post-certs-update triggers/post-certs-remove triggers/post-create triggers/post-delete triggers/report triggers/scheduler-app-status triggers/scheduler-deploy triggers/scheduler-enter triggers/scheduler-is-deployed triggers/scheduler-logs triggers/scheduler-proxy-config triggers/scheduler-proxy-logs triggers/scheduler-post-delete triggers/scheduler-run triggers/scheduler-run-list triggers/scheduler-stop triggers/scheduler-cron-write triggers/storage-create triggers/storage-destroy triggers/storage-status triggers/scheduler-storage-exec
+TRIGGERS = triggers/core-post-deploy triggers/core-post-extract triggers/install triggers/post-app-clone-setup triggers/post-app-rename-setup triggers/post-certs-update triggers/post-certs-remove triggers/post-create triggers/post-delete triggers/report triggers/scheduler-app-status triggers/scheduler-deploy triggers/scheduler-enter triggers/scheduler-is-deployed triggers/scheduler-logs triggers/scheduler-proxy-config triggers/scheduler-proxy-logs triggers/scheduler-post-delete triggers/scheduler-run triggers/scheduler-run-list triggers/scheduler-stop triggers/scheduler-cron-write triggers/scheduler-uses-host-cron triggers/storage-create triggers/storage-destroy triggers/storage-status triggers/scheduler-storage-exec
 BUILD = commands subcommands triggers
 PLUGIN_NAME = scheduler-k3s
 

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

plugins/scheduler-k3s/triggers.go

@@ -637,6 +637,17 @@ func TriggerSchedulerIsDeployed(scheduler string, appName string) error {
 	return fmt.Errorf("App %s is not deployed", appName)
 }
 
+// TriggerSchedulerUsesHostCron reports that the k3s scheduler does not use the
+// host crontab; it manages its own in-cluster CronJobs
+func TriggerSchedulerUsesHostCron(scheduler string) error {
+	if scheduler != "k3s" {
+		return nil
+	}
+
+	fmt.Println("false")
+	return nil
+}
+
 // TriggerSchedulerEnter enters a container for a given application
 func TriggerSchedulerEnter(scheduler string, appName string, processType string, podName string, args []string) error {
 	if scheduler != "k3s" {