I assume you can select whichever one you want (GPT-4o, o3-mini, Claude 3.5, 3.7, 3.7 thinking, Gemini 2.0 Flash, GPT=4.1 and the previews o1, Gemini 2.5 Pro and 04-mini), subject to the pricing multiplicators they announced recently [0].
Edit: From the TFA: Using the agent consumes GitHub Actions minutes and Copilot premium requests, starting from entitlements included with your plan.
[0] https://docs.github.com/en/copilot/managing-copilot/monitori...