feat: add devshell

devshell.nix

@@ -0,0 +1,35 @@
+{inputs, ...}: {
+  perSystem = {
+    pkgs,
+    system,
+    ...
+  }: {
+    devShells.default = pkgs.mkShell {
+      name = "nixos-config";
+      packages = with pkgs; [
+        # Nix tools
+        nil # Nix LSP
+        alejandra # Nix formatter
+        nix-diff # Compare derivations
+        nix-tree # Visualize dependencies
+        nvd # Nix version diff
+
+        # Secrets management
+        inputs.agenix.packages.${system}.default
+        age
+
+        # Deployment
+        just
+
+        # Git
+        git
+        jujutsu
+      ];
+
+      shellHook = ''
+        echo "🔧 NixOS Configuration Development Shell"
+        echo "   Run 'just' to see available commands"
+      '';
+    };
+  };
+}