Detect and treat terminated local/SSH/TTY shells as definitive command completion. Add is_terminated and get_exit_code helpers to LocalInteractiveSession, SSHInteractiveSession, and TTYSession; expand _is_closed_pty_error to recognize exited TTY processes. CodeExecution now reports a shell-exit prompt, recreates terminated sessions lazily before the next command, and returns immediately when a shell has exited. Docs and README updated to describe strict-mode/exit behavior, and tests were added/updated to cover the new termination detection and handling.
