Redesign the package

[nicholasjng]

New goals, prompted by better understanding of SWE practices, looks at successful projects, etc.:

1. Make the CLI a 1-2 command application at maximum. In particular, things like installs, venv management, worktrees, ... fall out of the responsibilities of the package.
2. Hand-roll GBM bindings with nanobind (to drop absl, get access to more functionality, ...)
3. Make a nicer presentation: rich text on the CLI, tab completion, ...
4. Support and third-party integrations like GH Actions, JSON sinks, and more.