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

devShell.nix

@@ -1,3 +1,4 @@
+{ inputs, ... }:
 {
   perSystem =
     {
@@ -8,6 +9,7 @@
     }:
     let
       writers = pkgs.callPackage ./pkgs/builders/script-writers.nix { };
+      inherit (pkgs.callPackage inputs.git-hooks { }) lib;
 
       ansiEscapes = {
         reset = ''\033[0m'';
@@ -20,6 +22,11 @@
       select-shell = writers.writePython3Bin "select-shell" {
         flakeIgnore = [ "E501" ];
       } ./pkgs/scripts/select-shell.py;
+
+      # run treefmt before each commit
+      install-pre-commit-hook =
+        with lib.git-hooks;
+        pre-commit (wrap.abort-on-change config.treefmt.build.wrapper);
     in
     {
       devShells.default = pkgs.mkShell {
@@ -34,8 +41,7 @@
           config.treefmt.build.wrapper
         ];
         shellHook = ''
-          # no longer used
-          rm -f "$(git rev-parse --show-toplevel)/.git/hooks/pre-commit"
+          ${install-pre-commit-hook}
 
           echo -e "${ansiEscapes.green}switch to another dev-shell using: select-shell${ansiEscapes.reset}"
         '';

flake.lock

@@ -40,6 +40,22 @@
         "type": "github"
       }
     },
+    "git-hooks": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1716413087,
+        "narHash": "sha256-nSTIB7JeJGBGsvtqlyfhUByh/isyK1nfOq2YMxUOFJQ=",
+        "owner": "fricklerhandwerk",
+        "repo": "git-hooks",
+        "rev": "99a78fcf7dc03ba7b1d5c00af109c1e28ced3490",
+        "type": "github"
+      },
+      "original": {
+        "owner": "fricklerhandwerk",
+        "repo": "git-hooks",
+        "type": "github"
+      }
+    },
     "nixlib": {
       "locked": {
         "lastModified": 1712450863,
@@ -133,6 +149,7 @@
       "inputs": {
         "disko": "disko",
         "flake-parts": "flake-parts",
+        "git-hooks": "git-hooks",
         "nixos-generators": "nixos-generators",
         "nixos-images": "nixos-images",
         "nixpkgs": "nixpkgs",

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 =