Proposed mechanism for "prompt variants". Added an "Aiden" prompt variant.

[deansher]

At least for my own purposes, I'd like to maintain a separate set of prompts that construct an assistant having a more sophisticated understanding of how to work with a "client".

This will cost tokens. Also, since this assistant is less eager to jump in and do the work, it likely won't benchmark as well. But I expect some will prefer it.

I wanted to make this change minimally invasive, so I can easily maintain it as the rest of the code evolves. Plus, stepping back from this particular set of prompts, I see lots of reasons for adding experimental or specialized assistants. If it sits well with you, I propose you incorporate both the mechanism and my "Aiden" prompt variant.

I haven't used this version of the assistant extensively yet. I've only tried it with 3.5 Sonnet. But I do find it easier to work with already, compared to "native" aider.