feat: add Home Manager as NixOS module and Flakes support

- flake.nix: new entrypoint using nixpkgs-unstable + home-manager input;
  builds as nixosConfigurations.chexit (nixos-rebuild switch --flake .#chexit)
- home.nix: Home Manager config for user chexit with zsh (p10k theme,
  autosuggestion, syntax highlighting, completion)
- shell.nix: stripped to system-level programs.zsh.enable only;
  user-specific zsh config moved to home.nix

https://claude.ai/code/session_01JwjdjwvgMGW4TcpYi4MZ6n

shell.nix

@@ -1,11 +1,5 @@
-{ pkgs, ... }:
+{ ... }:
 
 {
-  programs.zsh = {
-    enable = true;
-    enableCompletion = true;
-    autosuggestions.enable = true;
-    syntaxHighlighting.enable = true;
-    promptInit = "source ${pkgs.zsh-powerlevel10k}/share/zsh-powerlevel10k/powerlevel10k.zsh-theme";
-  };
+  programs.zsh.enable = true;
 }