Update package metadata and description

- Define `pname` once and inherit it in the package definition.
- Change description to “TUI underground RTS”.
- Set `mainProgram` to `pname` for consistency.

flake.nix

@@ -1,5 +1,5 @@
 {
-  description = "TUI underground game";
+  description = "TUI underground RTS";
 
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";