feat: initial v3 rewrite

nix/repo/devShells.nix

@@ -0,0 +1,27 @@
+{inputs, ...}: let
+  inherit (inputs) pkgs devshell treefmt;
+in {
+  default = devshell.mkShell {
+    packages = [
+      pkgs.nil
+      (treefmt.mkWrapper pkgs {
+        programs = {
+          alejandra.enable = true;
+          deadnix.enable = true;
+          statix.enable = true;
+          mdformat.enable = true;
+          yamlfmt.enable = true;
+        };
+        settings.formatter = {
+          yamlfmt.excludes = ["templates/nix-gitlab-ci.yml"];
+          mdformat.command = let
+            pkg = pkgs.python3.withPackages (p: [
+              p.mdformat
+              p.mdformat-mkdocs
+            ]);
+          in "${pkg}/bin/mdformat";
+        };
+      })
+    ];
+  };
+}