Add comprehensive inline documentation to both NixOS and Home Manager
global configuration files, explaining the purpose of each section,
overlay usage, unfree package policy, and configuration settings.
Move all allowUnfreePredicate declarations to global configs to prevent
the "last definition wins" merging issue. Unfree packages are now managed
in two central locations:
- NixOS system packages: nixos/common/global/default.nix
- Home Manager packages: home-manager/sajenim/global/default.nix