adding yubikey func

nixos/modules/yubikey.nix

@@ -0,0 +1,22 @@
+{ pkgs, ... }:
+
+{
+  # Create the plugdev group (required for user-space USB access to YubiKey)
+  users.groups.plugdev = {};
+
+  # Enable PC/SC daemon — required for GPG smartcard (gpg --card-status) and YubiKey
+  services.pcscd.enable = true;
+
+  # udev rules so the YubiKey gets plugdev group ownership and correct permissions
+  services.udev.packages = with pkgs; [
+    yubikey-personalization
+    libu2f-host
+  ];
+
+  # YubiKey management tools
+  environment.systemPackages = with pkgs; [
+    yubikey-manager          # ykman CLI
+    yubikey-personalization  # ykpers / ykchalresp
+    yubico-piv-tool          # PIV applet management
+  ];
+}