README package-manager copy buttons include descriptive comments

What is the issue?

The npm and Homebrew installation examples place their descriptive labels inside the shell code fences:

# Install using npm
npm install -g @openai/codex

GitHub's copy button therefore copies both the label and the executable command. Pasting the result does not produce a command-only snippet and may cause an error in environments where # is not treated as an interactive comment, such as a default interactive zsh configuration.

It would be clearer and safer to move the labels outside the fenced code blocks:

Using npm:

```shell
npm install -g @openai/codex
```

Using Homebrew:

```shell
brew install --cask codex
```

This keeps the rendered documentation equally clear while ensuring each copy button copies only the intended command.

Where did you find it?

https://github.com/openai/codex#installing-and-running-codex-cli

Open the repository README and inspect the “Installing and running Codex CLI” section linked in the issue. Move the npm and Homebrew labels outside their shell code fences, then verify the rendered copy buttons contain only the executable commands.