fix: an update reboots the computer only once nobody is using it

The update check has been daily at 03:00, Persistent, since 0.1.5 — the
lab's installed stable machine logged it at 03:01 and 03:04 on successive
days — and it stays so. What changes is the reboot after an update. It
happened the moment the update was installed: harmless at 03:00, and a
room restarting itself mid-morning when a computer switched off at 03:00
caught up after being switched on.

Now the updater installs, with every check it had (the device's own
channel, a newer version, the manifest's SHA-256, name and version), and
starts classroom-reboot-when-idle, which reboots once no input device has
produced an event for 30 minutes and no sound card has played for as
long. At 03:00 that is about 03:30; switched off first, the next start is
on the new version. The service has no [Install] and nothing enables it.

The release gate checks after every install and upgrade that the timer is
enabled, daily at 03:00, Persistent, with no boot or repeat trigger and
no drop-in, and that the updater no longer reboots by itself.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

tests/test-release-versions.sh

@@ -441,8 +441,11 @@ publish("canary", "0.2.7")
 status, out = run()
 check("a device on 0.2.6.1 recognises 0.2.7 as newer and installs it",
       status == 0 and installed() == "0.2.7", out)
-check("and reboots, because the manifest asks it to",
-      (work / "systemctl.log").read_text().strip() == "reboot")
+# Since 0.2.6.5 not there and then: the reboot is handed to a service that
+# waits until nobody is using the computer (classroom-reboot-when-idle).
+check("and asks for the reboot the manifest wants, once nobody is using the computer",
+      (work / "systemctl.log").read_text().strip()
+      == "start --no-block ekklesia-classroom-reboot-when-idle.service")
 publish("canary", "0.2.6.1")
 
 device("canary", "0.2.7")