Add support for OpenAI and Anthropic

[mbrt]

This PR adds support for models from Anthropic and OpenAI, in addition to the pre-existing Gemini models.

In addition:

• Add a new interactive init flow, which should make things much easier to jump start.
• Make configuration load a lot more intuitive, as: env is loaded first, default config next, and actual config at last, so defaults behave like actual defaults, but can always be overridden.
• Initial config is empty, so by default we can always evolve it transparently (unless users override).