feat(ghostty): enable SSH terminfo propagation and install terminfo on NixOS

Enable ssh-env and ssh-terminfo in Ghostty shell integration for automatic
terminfo handling on remote hosts, and add ghostty.terminfo to NixOS system
packages for coverage when Ghostty SSH wrapper is bypassed.

modules/nixos/base.nix

@@ -6,6 +6,7 @@
   environment.systemPackages = with pkgs; [
     curl
     git
+    ghostty.terminfo
     vim
     wget
   ];