feat(confing): dead scheduler config: config/schedule.rb points the whenever gem at a rake task that does not exist

config/schedule.yml

@@ -1,3 +1,6 @@
+# Schedules are owned by Sidekiq-cron and .ci-setup/crontab:
+# Sidekiq-cron owns in-process scheduled jobs.
+# .ci-setup/crontab owns the container's rake schedule.
 # config/schedule.yml
 
 register_webhooks:

test/config/scheduler_configuration_test.rb

@@ -0,0 +1,17 @@
+require "test_helper"
+
+class SchedulerConfigurationTest < ActiveSupport::TestCase
+  test "dead whenever schedule is removed" do
+    dead_schedule = Rails.root.join("config/schedule.rb")
+
+    refute dead_schedule.exist?,
+           "config/schedule.rb should be removed because whenever is not installed"
+  end
+
+  test "schedule owners are documented" do
+    schedule = Rails.root.join("config/schedule.yml").read
+
+    assert_includes schedule.lines.first, "Sidekiq-cron"
+    assert_includes schedule.lines.first, ".ci-setup/crontab"
+  end
+end