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/home/linux/apps/hexchat.nix

@@ -1,3 +1,4 @@
-{ ... }: {
+{ ... }:
+{
   programs.hexchat.enable = true;
 }