Adds a dialog (interactive + answerfile) letting the user choose whether
to copy the dotfiles' .zshrc / .bashrc / .vimrc into /etc/skel, or leave
system defaults in place. The choice is persisted as shell_rc in the
answerfile JSON and respected by both the TUI installer and the generator.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>