feat(nix): expose NixOS module and add CLI args for host/port

- Expose nixosModules.default in flake outputs for easy import
- Add argparse to main.py for --host and --port CLI flags
- Support priority: CLI args > env vars > defaults

flake.nix

@@ -18,8 +18,11 @@
     flake-utils,
     m3ta-nixpkgs,
     agents,
-  } @ inputs:
-    flake-utils.lib.eachDefaultSystem (system: let
+  }:
+    {
+      nixosModules.default = import ./nix/module.nix;
+    }
+    // flake-utils.lib.eachDefaultSystem (system: let
       pkgs = nixpkgs.legacyPackages.${system};
       pythonPackages = pkgs.python311Packages;