.backups folder may not be obvious to user and consumes disk space

**Link to original comment:** https://github.com/beyond-all-reason/BAR-Devtools/pull/33#discussion_r3367980676

Given that the .backups folder now has 23 GB of stuff in it... maybe we want a "just backup::purge" option surfaced to the user so they can see disk consumption done by the backup and easily delete backups that they no longer need.

Start by reading the linked discussion in pull request 33 and locate the existing backup command or the .backups handling it describes. Add a user-facing backup::purge option that shows backup disk usage and lets users delete backups they no longer need; done means the option is discoverable and safely removes only the intended backups.

Written by the indexing model from the issue text.