Add linting, formatting, and CI

- 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
- Format all nix files with nixfmt
- Add GitHub Actions workflow for CI validation
- Support x86_64-darwin in formatter

modules/shared/nixos/restic.nix

@@ -1,4 +1,5 @@
-{ config, pkgs, ... }: {
+{ config, pkgs, ... }:
+{
   environment.systemPackages = with pkgs; [
     restic
   ];
@@ -29,4 +30,3 @@
     };
   };
 }
-