README should note that codex app is macOS/Windows-only

What happened?

The README currently says:

If you want the desktop app experience, run <code>codex app</code> ...

This appears in the top-level guidance before the platform-specific install section. However, the app subcommand is compiled only on macOS and Windows:

• codex-rs/cli/src/main.rs gates app_cmd and desktop_app behind #[cfg(any(target_os = "macos", target_os = "windows"))].
• The Subcommand::App variant and handler are behind the same platform gate.

On Linux, the CLI source does not include the app subcommand, so the README can lead Linux users to try a command that is not available.

Expected behavior

The README should clarify that codex app is available only on supported desktop-app platforms, or direct unsupported platforms to the Codex App page without implying that the local command works everywhere.

Additional context

I searched open issues and PRs for terms including codex app, Linux, and README, and did not find an existing open item that appears to cover this documentation mismatch.