Centralize default policy configs

From discussion in #41:

We are starting to accumulate default literals for policy configurations (`algorithm='md5'`, `include_hidden=False`, `archive_dive=True`).

We should centralize these policies into a single `config.py` from which these and any new default policies can be imported.

(These are distinct though from e.g. `ignore_file=None` sentinel values.)

Start by locating the current uses of the default literals for algorithm, include_hidden, and archive_dive, then inspect the proposed config.py location. Move these policy defaults to the shared configuration and verify that existing behavior remains unchanged while new policy defaults can be imported from one place.

Written by the indexing model from the issue text.