docs: establish project design principles for contributors

[tmc]

As I've been doing some maintenance and improvements I've noticed some things creep in that we should clean up and codify in design principles.

A few topics this should cover:

• interface design
• package api design
• registry and provider patterns
• dependency policies

Another related document should cover the patterns and best practices employ when adding/extending which might include:

• test design / strategy

Here is a running scratchpad of things I'd like to see cleaned up and codified in some soft of automated checks:

• [ ] token counting mixed into llm package
• [ ] any model/provider specifics in any core package