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/xdg/niri/config.kdl

@@ -85,8 +85,7 @@ spawn-at-startup "nm-applet" "--indicator"
 spawn-sh-at-startup "waypaper --restore"
 // spawn-sh-at-startup "nemo . &> /dev/null &"
 spawn-sh-at-startup "foot --server"
-spawn-sh-at-startup "mkdir -p $XDG_RUNTIME_DIR/cliphist && wl-paste --watch cliphist -db-path $XDG_RUNTIME_DIR/cliphist/db store"
-spawn-sh-at-startup "mkdir -p $XDG_RUNTIME_DIR/cliphist && wl-paste --type image --watch cliphist -db-path $XDG_RUNTIME_DIR/cliphist/db store"
+// Clipboard history managed by cliphist.service (systemd user service)
 
 hotkey-overlay {
     hide-not-bound