set gpt-5 as default model for Windows users (#4676)

[min-mwei]

Codex isn’t great yet on Windows outside of WSL, and while we’ve merged openai#4269 to reduce the repetitive manual approvals on readonly commands, we’ve noticed that users seem to have more issues with GPT-5-Codex than with GPT-5 on Windows.

This change makes GPT-5 the default for Windows users while we continue to improve the CLI harness and model for GPT-5-Codex on Windows.

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