Design for extensibility via modules

Challange

Over time kas and agentk will accrue more and more features. I think it'd be great to put some structure in place to ensure that:

• Codebase evolves in the right direction.
• Adding and maintaining tests is easy.
• Adding new functionality is easy for us and somewhat easier for external contributors.

See this MR for the groundwork and refactoring of existing functionality into modules: !176 (merged). Note how much more structured and testable the code is now. (I still have to add more tests for things that are easier to test now and refactor some stuff for even greater testability)

Relates to #70 (closed).