feat(nix): silence the "Git tree is dirty" warning on rebuilds

The downstream flake at ~/.nomarchy is a live working tree by design —
nomarchy-theme-sync rewrites theme-state.json on every switch — so Nix's
dirty-tree warning fired on every sys-update/home-update and was just
noise. Default nix.settings.warn-dirty to false (mkDefault, overridable).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/nixos/default.nix

@@ -291,6 +291,11 @@ in
       settings = {
         experimental-features = [ "nix-command" "flakes" ];
         auto-optimise-store = lib.mkDefault true;
+        # The downstream flake (~/.nomarchy) is meant to be a live working
+        # tree: nomarchy-theme-sync rewrites theme-state.json on every
+        # switch (and you needn't commit each tweak), so the "Git tree is
+        # dirty" warning fires on every rebuild and is pure noise here.
+        warn-dirty = lib.mkDefault false;
       };
       gc = {
         automatic = lib.mkDefault true;