Add linting, formatting, and CI infrastructure

- Add deadnix, nixfmt, and statix to flake inputs
- Add formatter output to flake for nix fmt support
- Add deadnix, nixfmt, statix to Home Manager packages
- Add GitHub Actions workflow for CI validation
- Add .pre-commit-config.yaml with hooks for nixfmt, deadnix, and statix
- Support x86_64-darwin in formatter

modules/shared/nixos/internationalisation.nix

@@ -1,4 +1,4 @@
-{ ... }: {
+{
   # Select internationalisation properties.
   i18n = {
     defaultLocale = "en_US.UTF-8";
@@ -14,4 +14,4 @@
       LC_TIME = "en_US.UTF-8";
     };
   };
-}
+}