Add automatic garbage collection for home-manager user profiles

Configure nix.gc to automatically clean up old home-manager generations
weekly, which complements the existing system-level gc. Uses
--delete-older-than 10d on Linux to maintain a rollback window, but
overrides to -d on Darwin to work around a launchd bug where multi-part
options aren't properly split into separate arguments.

home/roles/base-darwin/default.nix

@@ -4,4 +4,8 @@
   imports = [
     ../aerospace
   ];
+
+  # Override to use -d instead of --delete-older-than on Darwin due to launchd bug
+  # https://github.com/nix-community/home-manager/issues/7211
+  nix.gc.options = "-d";
 }