Add an update command

What variant of Codex are you using?

CLI

What feature would you like to see?

I would love to see an update command to update the CLI tool itself. As a macOS user it's hard to keep track of how a tool was installed, e.g., brew, npm, uv, etc. A simple update command solves that issue perfectly. Copilot CLI for example has something similar that works well.

Additional information

_No response_