Extract shared NixOS/Darwin base config into roles/common.nix

Create roles/common.nix containing shared configuration between NixOS and
Darwin: timezone, base packages (git, glances, pciutils, tree, usbutils, vim),
nix settings (experimental-features, max-jobs, trusted-users), gc config,
and allowUnfree setting.

Both roles/default.nix and roles/darwin.nix now import common.nix and only
contain platform-specific configuration.

roles/default.nix

@@ -4,6 +4,7 @@ with lib;
 
 {
   imports = [
+    ./common.nix
     ./audio
     ./bluetooth
     ./btrfs
@@ -31,7 +32,6 @@ with lib;
       LC_TELEPHONE = "en_US.UTF-8";
       LC_TIME = "en_US.UTF-8";
     };
-    time.timeZone = "America/Los_Angeles";
 
     services.xserver.xkb = {
       layout = "us";
@@ -49,42 +49,7 @@ with lib;
     # Enable the OpenSSH daemon.
     services.openssh.enable = true;
 
-    environment.systemPackages = with pkgs; [
-      git
-      glances
-      pciutils
-      tree
-      usbutils
-      vim
-    ];
-
-    nix = {
-      package = pkgs.nix;
-      # distributedBuilds = true;
-      # buildMachines = [{
-      #   hostName = "z790prors.oglehome";
-      #   system = "x86_64-linux";
-      #   protocol = "ssh-ng";
-      #   sshUser = "johno";
-      #   sshKey = "/root/.ssh/id_ed25519";
-      #   maxJobs = 3;
-      #   speedFactor = 2;
-      # }];
-      settings = {
-        experimental-features = [ "nix-command" "flakes" ];
-        max-jobs = "auto";
-        trusted-users = [ "johno" ];
-        substituters = [
-        ];
-      };
-
-      gc = {
-        automatic = true;
-        randomizedDelaySec = "14m";
-        options = "--delete-older-than 10d";
-      };
-    };
-
-    nixpkgs.config.allowUnfree = true;
+    # NixOS-specific gc option (not available on Darwin)
+    nix.gc.randomizedDelaySec = "14m";
   };
 }