feat: add cliphist service, move user services to /etc/systemd/user/

- Add cliphist.service that wipes clipboard history on session start
  (crash-safe: cleans up at next boot, not at shutdown)
- Move kanshi, walker, nautilus services from ~/.config/systemd/user/
  to /etc/systemd/user/ (system-level defaults, consistent with
  moonarch's config philosophy)
- Remove cliphist spawn-sh-at-startup from niri config (managed by
  systemd service now)
- Add cliphist to USER_SERVICES in post-install.sh and transform.sh

Closes #1

defaults/etc/systemd/user/nautilus.service

@@ -0,0 +1,11 @@
+[Unit]
+Description=Nautilus File Manager (GApplication Service)
+After=graphical-session.target
+
+[Service]
+Type=simple
+ExecStart=nautilus --gapplication-service
+Restart=on-failure
+
+[Install]
+WantedBy=graphical-session.target