feat(ssh): add enable option

modules/ssh/default.nix

@@ -1,10 +1,17 @@
-{ lib, ... }:
+{ lib, config, ... }:
 
+let
+  cfg = config.ssh;
+in
 {
   imports = [ ./hardened-openssh.nix ];
 
-  config.services.openssh = {
-    enable = lib.mkDefault true;
-    harden = lib.mkDefault true;
+  options.ssh.enable = lib.mkEnableOption "SSH server";
+
+  config = lib.mkIf cfg.enable {
+    services.openssh = {
+      enable = lib.mkDefault true;
+      harden = lib.mkDefault true;
+    };
   };
 }