sessions delete and prune call input() for confirmation without
catching EOFError. When stdin isn't a TTY (piped input, CI/CD, cron),
input() throws EOFError and the command crashes.
Extract a _confirm_prompt() helper that handles EOFError and
KeyboardInterrupt, defaulting to cancel. Both call sites now use it.
Salvaged from PR #2622 by dieutx (improved from duplicated try/except
to shared helper). Closes#2565.