make sure things are sane before they hit CI, re-purposing the existing
treefmt configuration.
this adds a custom installer for pre-commit hooks, which is inspired by
pre-commit.nix[0], but is much more minimal than the underlying
pre-commit[1] and builds on a historic idea[2] from this repository.
[0]: https://github.com/cachix/git-hooks.nix
[1]: https://github.com/pre-commit/pre-commit
[2]: 930923512c
