add pre-commit check

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

flake.nix

@@ -21,6 +21,8 @@
     flake-parts.inputs.nixpkgs-lib.follows = "nixpkgs";
     treefmt-nix.url = "github:numtide/treefmt-nix";
     treefmt-nix.inputs.nixpkgs.follows = "nixpkgs";
+    git-hooks.url = "github:fricklerhandwerk/git-hooks";
+    git-hooks.flake = false;
   };
 
   outputs =