require a configuration file (abort if there isn't one.)

replacing a note in the README. Results of just using defaults in the absence of a config file can be surprising. We should figure out how to complain effectively about this, rather than doing something unexpected.

Start with the README note and trace the command-line entry point to find where configuration loading and defaults are handled. Confirm how the program behaves without a configuration file and identify the existing error-reporting path. Done means the absence of a configuration file is detected and reported clearly before unexpected default behavior occurs.

Written by the indexing model from the issue text.