Add linting, formatting, and CI with fixes for all warnings

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

Statix fixes (W10/W20 warnings):
- Remove unused lambda argument from nixpkgs-settings.nix
- Merge repeated keys in hardware-configuration.nix files (boot.initrd, boot, fileSystems)
- Merge repeated keys in nixnuc/default.nix (services, virtualisation)
- Merge repeated keys in rainbow-planet/default.nix (desktopManager)
- Merge repeated keys in home/general/default.nix (home)

Deadnix fixes (unused declarations):
- Remove unused pkgs/lib/username/http_port arguments from various files
- Fix unused final parameter in overlay functions (final -> _final)

CI/pre-commit fixes:
- Fix pre-commit statix config: add pass_filenames: false
- Fix CI workflow: use nix run nixpkgs# prefix and --ci flag for nixfmt

modules/hosts/home-manager-only/default.nix

@@ -1,4 +1,11 @@
-{ config, pkgs, system, username, ... }: {
+{
+  config,
+  pkgs,
+  system,
+  username,
+  ...
+}:
+{
   home.stateVersion = "25.05";
   home.packages = with pkgs; [
     age

modules/hosts/home-manager-only/home-gene.liverman.nix

@@ -1,3 +1,3 @@
-{ ... }: {
+{
   # Settings just for work machines go here
-}
+}

modules/hosts/home-manager-only/home-gene.nix

@@ -1,4 +1,5 @@
-{ pkgs, ... }: {
+{ pkgs, ... }:
+{
   # Settings just for personal machines go here
 
   home.packages = with pkgs; [