feat: Add multi-provider support (GPT, Gemini, etc.)

[Octane0411]

Hey! Thanks for this repo, learned a lot from it. I noticed it's Anthropic-only right now. I don't actually have an Anthropic key, so I hacked together an OpenAI adapter to make it work with other APIs while I was learning. Figured others might have the same issue, so it'd be cool to support other providers out of the box so anyone can just jump in. Works fine so far with my Gemini API, maybe I'll raise a PR later.