chore(nix): remove statix and deadnix from fixer

Drop the statix and deadnix packages from the fix app now that the repository has a local Nix style linter path. Remove the obsolete statix configuration, document the evaluated rule sets and architectures, and keep vendored third-party files out of treefmt.

src/dotfiles/apps.nix

@@ -15,14 +15,9 @@ in
         name = "fix";
         runtimeInputs = [
           formatter
-          pkgs.statix
-          pkgs.deadnix
         ];
         text = ''
           treefmt
-          statix fix .
-          deadnix --edit --exclude configs/nixos/hosts/*/hardware-configuration.nix -- .
-          treefmt
         '';
       }
     );

src/dotfiles/formatter.nix

@@ -6,5 +6,6 @@ inputs.treefmt-nix.lib.evalModule inputs.nixpkgs.legacyPackages.${system} {
 
   settings.formatter.nixfmt.excludes = [
     "configs/nixos/hosts/*/hardware-configuration.nix"
+    "third_party/**"
   ];
 }