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.

README.md

@@ -26,6 +26,7 @@
 - [Funding Opportunity](#funding-opportunity)
 - [Contributing](#contributing)
   - [Development workflow](#development-workflow)
+    - [Nix Flake Development](#nix-flake-development)
   - [Writing high‑impact code changes](#writing-highimpact-code-changes)
   - [Opening a pull request](#opening-a-pull-request)
   - [Review process](#review-process)
@@ -412,6 +413,31 @@ npm run lint:fix
 npm run format:fix
 ```
 
+#### Nix Flake Development
+
+Prerequisite: Nix >= 2.4 with flakes enabled (`experimental-features = nix-command flakes` in `~/.config/nix/nix.conf`).
+
+Enter a Nix development shell:
+
+```bash
+nix develop
+```
+
+This shell includes Node.js, installs dependencies, builds the CLI, and provides a `codex` command alias.
+
+Build and run the CLI directly:
+
+```bash
+nix build
+./result/bin/codex --help
+```
+
+Run the CLI via the flake app:
+
+```bash
+nix run .#codex
+```
+
 ### Writing high‑impact code changes
 
 1. **Start with an issue.** Open a new one or comment on an existing discussion so we can agree on the solution before code is written.