refactor: move max into dentritic pattern

modules/shell.nix

@@ -0,0 +1,19 @@
+{ ... }: {
+  perSystem = { pkgs, ... }: {
+    devShells.default = pkgs.mkShell {
+      shellHook = ''
+        export SOPS_AGE_KEY="$(${pkgs.age-plugin-yubikey}/bin/age-plugin-yubikey --identity)"
+      '';
+
+      packages = with pkgs; [
+        age
+        ssh-to-age
+        sops
+        just
+        nix-inspect
+
+        age-plugin-yubikey
+      ];
+    };
+  };
+}