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.

.gitignore

@@ -10,6 +10,9 @@ storybook-static/
 # ignore README for publishing
 codex-cli/README.md
 
+# ignore Nix derivation results
+result
+
 # editor
 .vscode/
 .idea/