Design system MCP: Document Codex CLI prerequisite for MCP setup

[mirka]

What?

Adds a note to the design system MCP setup instructions clarifying that OpenAI Codex users need the Codex CLI installed before running codex mcp add.

Why?

It might not be obvious to a non-technical reader.

Testing Instructions

1. Open packages/design-system-mcp/README.md.
2. Confirm the OpenAI Codex section mentions the Codex CLI prerequisite and links to the setup guide.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: mirka <0mirka00@git.wordpress.org>
Co-authored-by: aduth <aduth@git.wordpress.org>
Co-authored-by: ciampo <mciampini@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[aduth]

I'm not as familiar with the Codex family of tools, but would it also work to append "CLI" in the subheading to clarify? It feels like a similar distinction between Claude Code and Claude Desktop, and we don't document that "Claude Code must be installed first" in the section about it. Or maybe we should, just to be extra consistent between the app and CLI variants of these tools 😄

[mirka]

My understanding was that the CLI tooling provides a convenient way to add the MCP server to the Codex desktop app config as well, not just configure it for the CLI itself. Is that correct @ciampo?

[ciampo]

AFAIK, the Codex app doesn't add codex to the PATH. In order to have that (and run the mcp add command) a users need to install the CLI.

Users can also install an MCP server via the app's GUI by essentially filling a form.

For example, this is is what I see in the Codex app after running the CLI command — essentially, they're just two different ways to set the same settings

[mirka]

Thanks for clarifying. It's probably reasonable to just document the CLI way, as it's most convenient and unlikely to get outdated due to app-side changes.