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.

update.sh

@@ -43,6 +43,13 @@ fi
 
 CHANNEL="$(tr -d '[:space:]' < "$CHANNEL_FILE")"
 
+# ── Detect interactive stdin ──────────────────────────────────────────────────
+if [ -t 0 ]; then
+    IS_INTERACTIVE=true
+else
+    IS_INTERACTIVE=false
+fi
+
 # ── Colours (if terminal) ─────────────────────────────────────────────────────
 if [ -t 1 ]; then
     RED='\033[0;31m'
@@ -96,9 +103,15 @@ STOPPED=0   # set once the service has been stopped
 STARTED=0   # set once it has been (re)started
 
 # ── Stop service ──────────────────────────────────────────────────────────────
+# NOTE: match the *normalized* OS values set above (linux/darwin), not uname's
+# capitalized output. Getting this wrong turns both stop_service and
+# start_service into silent no-ops, and then none of the ordering this file
+# documents at the top actually happens: the tarball is extracted over the
+# running binary (ETXTBSY on Linux, aborting the update mid-way), and the
+# safety-net restart in cleanup() is a no-op too, so the box stays down.
 stop_service() {
     case "$OS" in
-        Linux)
+        linux)
             if command -v systemctl >/dev/null 2>&1; then
                 if systemctl --user is-active skald-circle.service >/dev/null 2>&1; then
                     info "⏹️  Stopping service …"
@@ -106,7 +119,7 @@ stop_service() {
                 fi
             fi
             ;;
-        Darwin)
+        darwin)
             if command -v launchctl >/dev/null 2>&1; then
                 if launchctl list com.skald.circle >/dev/null 2>&1; then
                     info "⏹️  Stopping agent …"
@@ -145,13 +158,13 @@ wait_until_stopped() {
 # ── Start service ─────────────────────────────────────────────────────────────
 start_service() {
     case "$OS" in
-        Linux)
+        linux)
             if command -v systemctl >/dev/null 2>&1; then
                 info "▶ Starting service …"
                 systemctl --user start skald-circle.service
             fi
             ;;
-        Darwin)
+        darwin)
             if command -v launchctl >/dev/null 2>&1; then
                 info "▶ Starting agent …"
                 launchctl load "$HOME/Library/LaunchAgents/com.skald.circle.plist" 2>/dev/null || true
@@ -160,6 +173,34 @@ start_service() {
     esac
 }
 
+# ── systemd user lingering ────────────────────────────────────────────────────
+# Same helper the installers run, repeated here so an install predating it gets
+# healed by an ordinary update: a `systemctl --user` unit lives under the
+# per-user manager, which systemd stops when the user's last session ends —
+# so without lingering the server dies at logout and never starts at boot.
+# Idempotent, and a failure is only ever a warning: the update itself is fine.
+ensure_linger() {
+    [ "$OS" = "linux" ] || return 0
+
+    local target="${USER:-$(id -un)}"
+
+    command -v loginctl >/dev/null 2>&1 || return 0
+
+    case "$(loginctl show-user "$target" --property=Linger 2>/dev/null || true)" in
+        *=yes) return 0 ;;
+    esac
+
+    if loginctl enable-linger "$target" 2>/dev/null \
+        || sudo -n loginctl enable-linger "$target" 2>/dev/null \
+        || { [ "$IS_INTERACTIVE" = true ] && sudo loginctl enable-linger "$target"; }; then
+        info "✔ Lingering enabled — the server now survives logout and starts at boot"
+    else
+        warn "Lingering is not enabled for ${target}."
+        echo "  The server stops when your last session ends. Run this once:"
+        echo "      sudo loginctl enable-linger ${target}"
+    fi
+}
+
 # ── Cleanup + safety net ──────────────────────────────────────────────────────
 # Runs on every exit. Removes temp files and, if the update died after the
 # service was stopped but before it came back up, makes a best-effort restart so
@@ -279,6 +320,7 @@ main() {
     fi
 
     # ── Restart ────────────────────────────────────────────────────────────────
+    ensure_linger
     start_service
     STARTED=1