[RFC] Interface for `croco check` command

It would be nice to have a command that can tell which repositories need to be updated, which repositories are missing files from the template repository and general files. Probably configuration format should be updated to support a list of files to check. So this issue is to discuss potential interface and features for this command.

Start by reviewing the proposed `croco check` command and the current configuration format. Define the interface for reporting repositories needing updates, repositories missing template files, and missing general files, including how a list of files is configured. Done means the command's scope and configuration format are agreed.

Written by the indexing model from the issue text.