Prevent silent hangs on non-interactive stdin

Add isatty() checks to prompt_yn() and update variant chooser.
When stdin is not a terminal and no -y/-n or --standard/--ui flag
is provided, print a clear error and exit instead of hanging on
fgets(). Prevents agents from silently blocking when running
install/update programmatically.

Add smoke test 9b-9: verify non-interactive update fails cleanly.