Model versions and preview

Guidance currently leans on models that are in GA. Should probably make a decision about whether to include preview models more clearly, but I think this is a smaller issue for someone new to agents. Just something to make sure we're capturing clearly.

e.g. You'll notice a lot of [Gemini 2.x mentions](https://github.com/search?q=repo%3Aaddyosmani%2Fagent-engineer%20gemini&type=code) in the repo vs Gemini 3.x.