refactor(nix): move style app wiring into dotfiles

src/dotfiles/apps.nix

@@ -0,0 +1,40 @@
+{
+  inputs,
+  lib,
+}:
+let
+  system = "x86_64-linux";
+  pkgs = inputs.nixpkgs.legacyPackages.${system};
+
+  treefmtEval = inputs.treefmt-nix.lib.evalModule pkgs {
+    projectRootFile = "flake.nix";
+
+    programs.nixfmt.enable = true;
+
+    settings.formatter.nixfmt.excludes = [
+      "configs/nixos/hosts/*/hardware-configuration.nix"
+    ];
+  };
+
+  styleCheck = pkgs.writeShellApplication {
+    name = "style-check";
+    runtimeInputs = [
+      treefmtEval.config.build.wrapper
+      pkgs.statix
+      pkgs.deadnix
+    ];
+    text = ''
+      treefmt --fail-on-change
+      statix check .
+      deadnix --fail --no-lambda-pattern-names --no-underscore .
+    '';
+  };
+in
+{
+  formatter.${system} = treefmtEval.config.build.wrapper;
+
+  apps.${system}.style-check = {
+    type = "app";
+    program = lib.getExe styleCheck;
+  };
+}