Add option to set log file save location

> I was surprised that the checksum CSV and log files created in my Downloads directory: `/Users/john/Downloads/burress_checksums.csv` and `/Users/john/Downloads/burress_log.jsonl`.
It would be nice to have control over where these files get written.
It also seems that these files get overwritten if run again.

> Could you create an issue to determine the best location to write these log/metadata files and possibly prompt before overwriting them?

_Originally posted by @johnbradley in https://github.com/Imageomics/cautious-robot/issues/1#issuecomment-2149743061_

Start by locating where burress_checksums.csv and burress_log.jsonl are written, then inspect how the downloader obtains its output location. Clarify the desired default location, configuration interface, and overwrite or prompt behavior before implementing. Done means users can control the destination and repeated runs have defined overwrite behavior.

Written by the indexing model from the issue text.