optimize flake shape and add devshell

modules/devshells.nix

@@ -0,0 +1,23 @@
+{ ... }:
+{
+  perSystem =
+    { pkgs, ... }:
+    {
+      formatter = pkgs.nixfmt-tree;
+
+      devShells.default = pkgs.mkShell {
+        packages = with pkgs; [
+          deadnix
+          git
+          just
+          nixfmt-tree
+          nodePackages.prettier
+          pre-commit
+          selene
+          shfmt
+          statix
+          stylua
+        ];
+      };
+    };
+}