This change swaps the plain stdin loop for a rustyline-based editor, giving the shell a more usable interactive experience with line editing, persistent history behavior, and tab completion for built-ins like echo, exit, type, pwd, and cd. It also introduces completion tests and fixes command parsing so quoted filenames in redirections are unquoted correctly, addressing a real usability bug alongside the UX upgrade. The practical effect is a shell that feels much closer to a real terminal and behaves more reliably during everyday use.