Reproducible and automatically configure development environments

[expelledboy]

To rapidly onboard new developers its best to provide a reproducible development environment.

This PR achieves the following:

• deterministic system dependencies using nixpkgs via ./flake.nix
• automatic repo setup via shellHook in ./flake.nix using cached direnv via nix-direnv
• lint checks using ruff both as git pre-commit and .github/workflow/test.yml for PRs and merges to master
• identified and provided repo tasks via Justfile (only 2 atm, but likely to grow)

Concerns

I couldnt download the 300+ GB model, so havent tested the download-weights task.