Restructure flake into per-host modules

Move host-specific files under hosts/<name>/ so a second machine can be
added without forking the repo. Split system/users.nix and
system/display-manager.nix into per-option subfolders, separate the
home-manager packages list into a shared base and a personal bundle,
extract networking.hostName out of the shared networking module, and
inline the git identity into the per-host home.nix. Add .gitignore
guardrails to keep secrets out.

home/zsh.nix

@@ -19,7 +19,7 @@
 
     shellAliases = {
       btw = "echo I use NixOS";
-      reload-nixos = "sudo nixos-rebuild switch --flake ~/nixos-dotfiles#hivebrain";
+      reload-nixos = "sudo nixos-rebuild switch --flake ~/nixos-dotfiles";
     };
   };
 }