feat: add Nix flake for reproducible development environments (#225)

This PR introduces a Nix flake configuration to enable reproducible
development environments:

- Adds flake.nix defining a devShell with necessary dependencies.
- Updates README.md with usage instructions for `nix develop`.
- Ensures CI compatibility with Nix for consistent builds.

flake.nix

@@ -0,0 +1,51 @@
+{
+  description = "Development Nix flake for OpenAI Codex CLI";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils, ... }: 
+    flake-utils.lib.eachDefaultSystem (system: let
+      pkgs = import nixpkgs { inherit system; };
+      node = pkgs.nodejs_22;
+    in rec {
+      packages = {
+        codex-cli = pkgs.buildNpmPackage rec {
+          pname       = "codex-cli";
+          version     = "0.1.0";
+          src         = self + "/codex-cli";
+          npmDepsHash = "sha256-riVXC7T9zgUBUazH5Wq7+MjU1FepLkp9kHLSq+ZVqbs=";
+          nodejs      = node;
+          npmInstallFlags = [ "--frozen-lockfile" ];
+          meta = with pkgs.lib; {
+            description = "OpenAI Codex command‑line interface";
+            license     = licenses.asl20;
+            homepage    = "https://github.com/openai/codex";
+          };
+        };
+      };
+      defaultPackage = packages.codex-cli;
+      devShell = pkgs.mkShell {
+        name        = "codex-cli-dev";
+        buildInputs = [
+          node
+        ];
+        shellHook = ''
+          echo "Entering development shell for codex-cli"
+          cd codex-cli
+          npm ci
+          npm run build
+          export PATH=$PWD/node_modules/.bin:$PATH
+          alias codex="node $PWD/dist/cli.js"
+        '';
+      };
+      apps = {
+        codex = {
+          type    = "app";
+          program = "${packages.codex-cli}/bin/codex";
+        };
+      };
+    });
+}