ghostty: prevent service restarts

Stopping or restarting the service will destroy open Ghostty sessions,
potentially disrupting work. This change ensures that we leave the
existing service untouched.

Fixes #8485

tests/modules/programs/ghostty/example-settings.nix

@@ -11,6 +11,9 @@
   };
 
   nmt.script = ''
+    servicePath=home-files/.config/systemd/user/app-com.mitchellh.ghostty.service
+    assertPathNotExists $servicePath
+
     assertFileContent \
       home-files/.config/ghostty/config \
       ${./example-config-expected}