Add nix shell config

shell.nix

@@ -0,0 +1,17 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  packages = with pkgs; [
+    python312
+    python312Packages.pip
+    python312Packages.virtualenv
+  ];
+
+  shellHook = ''
+    if [ ! -d .venv ]; then
+      python -m venv .venv
+    fi
+    source .venv/bin/activate
+    pip install -r requirements.txt
+  '';
+}