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

scripts/post-install.sh

@@ -151,6 +151,7 @@ log "Enabling systemd user services..."
 USER_SERVICES=(
     "kanshi"
     "stasis"
+    "cliphist"
 )
 
 for service in "${USER_SERVICES[@]}"; do

scripts/transform.sh

@@ -344,6 +344,7 @@ log "Enabling systemd user services..."
 USER_SERVICES=(
     "kanshi"
     "stasis"
+    "cliphist"
 )
 
 for service in "${USER_SERVICES[@]}"; do