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]>

tools/build-release.sh

@@ -193,6 +193,10 @@ install -Dm755 \
     "$ROOT/scripts/ekklesia-classroom-update" \
     "$STAGE/usr/local/bin/ekklesia-classroom-update"
 
+install -Dm755 \
+    "$ROOT/scripts/classroom-reboot-when-idle" \
+    "$STAGE/usr/local/bin/classroom-reboot-when-idle"
+
 # System configuration.
 install -Dm644 \
     "$ROOT/config/sddm/ekklesia-classroom.conf" \
@@ -206,6 +210,11 @@ install -Dm644 \
     "$ROOT/systemd/updater/ekklesia-classroom-update.timer" \
     "$STAGE/etc/systemd/system/ekklesia-classroom-update.timer"
 
+# After an update: the reboot, once nobody is using the computer.
+install -Dm644 \
+    "$ROOT/systemd/updater/ekklesia-classroom-reboot-when-idle.service" \
+    "$STAGE/etc/systemd/system/ekklesia-classroom-reboot-when-idle.service"
+
 install -Dm644 \
     "$ROOT/systemd/provision/ekklesia-classroom-provision.service" \
     "$STAGE/etc/systemd/system/ekklesia-classroom-provision.service"