fix: keep the server running after you log out

A `systemctl --user` unit runs under the per-user manager, which systemd
starts at first login and stops when the user's last session ends — so
closing the SSH session that started Skald killed it, and it never came
up at boot. No crash and nothing in the journal: the whole cgroup is
simply torn down. Both installers now enable lingering after installing
the unit, and update.sh carries the same helper so an installation
predating this fix is healed by an ordinary update. A failure to enable
it only ever warns, with the manual command — it must not abort an
install.

Two things found on the way there:

update.sh matched `case "$OS" in Linux) ... Darwin)`, but $OS had already
been normalized to lowercase at the top of the file, so stop_service and
start_service were both silent no-ops. None of the ordering the file
documents at its head was executing: the tarball went over the running
binary (ETXTBSY, aborting the update mid-way) and the safety-net restart
in cleanup() was a no-op too, leaving the box down.

Neither workflow published install.sh / install-nightly.sh to the web
root, so the scripts served by builds.skaldagent.net were hand-copied and
drifting from the repo — an installer fix would reach every existing box
through update.sh but never a new one. Nightly publishes the nightly
installer, release publishes the release one, both with the same atomic
temp-and-rename the tarballs use.

Also on the unit: dropped `After=docker.service`, which a user manager
silently ignores rather than honouring advisorily, and moved
`Restart=on-failure` to `always` — run.sh exits 0 on any graceful
shutdown, including one nobody asked for, which on-failure reads as a
clean stop. That is also what absorbs the boot race against Docker now
that lingering makes us start at boot.

SKALD.md

@@ -98,6 +98,22 @@ systemd service → ExecStart=run.sh
 
 **Fix**: removed `Requires=docker.service` from the user unit template in both install scripts. Kept `After=docker.service` (advisory, doesn't block if the unit isn't found).
 
+**Follow-up**: `After=docker.service` was dropped too. It never did anything — a _user_ manager has no view of system units, so the ordering was silently ignored rather than merely advisory, and keeping it suggested a guarantee that was not there. What actually handles the boot race is `Restart` (see below): the server fails fast when the Docker daemon is unreachable, and systemd brings it back a few seconds later.
+
+## Bug fix: the server dies when you log out ✅
+
+**Problem**: `systemctl --user start skald-circle` worked, but closing the SSH session killed the server — and it never came up at boot. Not an application bug: a `--user` unit runs under the per-user manager (`user@UID.service`), which systemd starts at first login and **stops when the user's last session ends**, tearing down every user service in the cgroup. No crash, no error in the journal — the whole cgroup is simply killed.
+
+**Fix**: both installers now run `loginctl enable-linger $USER` after installing the unit (helper `enable_linger`, tried unprivileged first, then `sudo -n`, then interactive `sudo`, and only warns if all three fail — a missing linger must never abort an install). `update.sh` carries the same helper so an installation predating this fix is healed by an ordinary update.
+
+**Also**: `Restart=on-failure` → `Restart=always`. `run.sh` exits 0 on _any_ graceful shutdown, including one nobody asked for (a stray SIGTERM to the server), which `on-failure` reads as a clean stop and leaves the box down. An explicit `systemctl --user stop` is unaffected — systemd never restarts after a requested stop. With lingering on, this is also what absorbs the boot race against Docker.
+
+## Bug fix: update.sh never stopped or restarted the service ✅
+
+**Problem**: `stop_service` and `start_service` matched `case "$OS" in Linux) … Darwin)`, but `$OS` had already been normalized to `linux`/`darwin` at the top of the script. Every branch fell through: both functions were no-ops. So the updater extracted the tarball **over the running binary** (`ETXTBSY` on Linux, aborting the update mid-way) and, when extraction did succeed, left the old build running in memory with the safety-net trap firing a restart that was itself a no-op. The careful stop → wait-for-exit → extract ordering the file documents at the top had not been executing at all.
+
+**Fix**: matched the normalized lowercase values, with a comment at the seam saying why the capitalization is load-bearing.
+
 ## Bug fix: skald-setup non interattivo con curl | bash ✅
 
 **Problem**: `skald-setup` controlla `isatty(0)`, ma con `curl ... | bash` stdin è un pipe, quindi saltava senza chiedere username/password. L'installer arrivava fino in fondo ma senza aver creato l'admin.