switch over hopper fully (+ minor refactoring)

nix/systemProfiles/core/ssh.nix

@@ -0,0 +1,22 @@
+{lib, ...}: {
+  services.openssh = {
+    enable = lib.mkDefault true;
+    settings = {
+      # Use only public keys
+      PasswordAuthentication = lib.mkForce false;
+      KbdInteractiveAuthentication = lib.mkForce false;
+
+      # root login is never welcome, except for remote builders
+      PermitRootLogin = lib.mkForce "prohibit-password";
+    };
+
+    startWhenNeeded = lib.mkDefault true;
+    openFirewall = lib.mkDefault true;
+    hostKeys = [
+      {
+        path = "/etc/ssh/ssh_host_ed25519_key";
+        type = "ed25519";
+      }
+    ];
+  };
+}