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

flake.nix

@@ -0,0 +1,26 @@
+{
+  description = "chexit NixOS configuration";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    home-manager = {
+      url = "github:nix-community/home-manager";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs = { nixpkgs, home-manager, ... }: {
+    nixosConfigurations.chexit = nixpkgs.lib.nixosSystem {
+      system = "x86_64-linux";
+      modules = [
+        ./configuration.nix
+        home-manager.nixosModules.home-manager
+        {
+          home-manager.useGlobalPkgs = true;
+          home-manager.useUserPackages = true;
+          home-manager.users.chexit = import ./home.nix;
+        }
+      ];
+    };
+  };
+}