feat(jeeves): wireguard

Add separate module for `wireguard`
Rekey `jeeves_password` (use all `users`' keys)
Add secrets related to Wireguard: server public/private, cheetah public
Add a `.gitignore` for the private `limonka_age` key

machines/nixos/x86_64-linux/jeeves/configuration.nix

@@ -4,6 +4,7 @@
     (import ./disko.nix { inherit inputs outputs; })
     inputs.agenix.nixosModules.default
     ./network.nix
+    ./wireguard.nix
   ];
 
   nixpkgs = {