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";
 }

home/roles/base/default.nix

@@ -24,6 +24,17 @@ in
       tree
     ];
 
+    # Automatic garbage collection for user profile (home-manager generations).
+    # This complements system-level gc which only cleans system generations.
+    # - Linux: Uses --delete-older-than to keep 10-day rollback window
+    # - Darwin: Overridden to use -d in base-darwin role to avoid launchd bug
+    #   (https://github.com/nix-community/home-manager/issues/7211)
+    nix.gc = {
+      automatic = true;
+      randomizedDelaySec = mkIf pkgs.stdenv.isLinux "14m";
+      options = "--delete-older-than 10d";
+    };
+
     # Essential programs everyone needs
     programs.bash = {
       enable = true;