feat(bin): support bun fallback runtime for codex CLI (#282)

This PR adds a shell wrapper in `codex-cli/bin/codex` to detect node or
bun as the runtime.

It updates:
- `package.json` bin entry
- published files list to include bin/
- README install instructions to include `bun install -g @openai/codex`

README.md

@@ -241,6 +241,8 @@ Below are a few bite‑size examples you can copy‑paste. Replace the text in q
 npm install -g @openai/codex
 # or
 yarn global add @openai/codex
+# or
+bun install -g @openai/codex
 ```
 
 </details>