feat: add systemd service config and install script

config/systemd/user/claude-sessions-save.service: source of truth
for the shutdown service, version-controlled alongside the scripts
it calls.

scripts/install-systemd.sh: idempotent install script — copies the
service file to ~/.config/systemd/user/, reloads the daemon, and
enables + starts the service. Run on any new machine to wire up
graceful Claude session shutdown.

config/systemd/user/claude-sessions-save.service

@@ -0,0 +1,18 @@
+[Unit]
+Description=Gracefully exit Claude Code sessions before shutdown
+Documentation=https://github.com/oreillyit/claude-foundations
+# Run before the system begins tearing down the session
+Before=shutdown.target halt.target reboot.target poweroff.target
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+# ExecStart is a no-op — the service exists purely for its ExecStop
+ExecStart=/bin/true
+# On stop (triggered at shutdown): gracefully exit all Claude tmux sessions
+ExecStop=%h/dev/claude/projects/claude-foundations/scripts/claude-sessions-shutdown
+# Allow up to 120s for Claude sessions to save and exit cleanly
+TimeoutStopSec=120
+
+[Install]
+WantedBy=default.target