Fixes#9822
### Summary
Make the Homebrew upgrade command explicit by using `brew upgrade --cask
codex`.
### Motivation
During the Codex self-update, Homebrew can emit an avoidable warning
because the
name `codex` resolves to a cask:
```
Warning: Formula codex was renamed to homebrew/cask/codex.
````
While the upgrade succeeds, this relies on implicit name resolution and
produces
unnecessary output during the update flow.
### Why `--cask`
* Eliminates warning/noise for users
* Explicitly matches how Codex is distributed via Homebrew
* Avoids reliance on name resolution behavior
* Makes the command more robust if a `codex` formula is ever introduced
### Context
This restores the `--cask` flag that was removed in #6238 after being
considered
“not necessary” during review:
[https://github.com/openai/codex/pull/6238#discussion_r2505947880](https://github.com/openai/codex/pull/6238#discussion_r2505947880).
Co-authored-by: Eric Traut <etraut@openai.com>