Keep track of parent repo

For illustration, my projects look like this

https://github.com/python-packaging/skel

https://github.com/thatch/imperfect (branches main, skel)
local clone (main, skel) referring to (origin/main, origin/skel)

Those two github urls do not share any common ancestors; the repo skel has the code and templates to generate the branch skel. If someone wants a different default license, or changes to defaults (like whether to use codecov.io), they fork that repo.

Towards and eventual goal of e.g. a skelup command/service that runs regen-git for you against a checkout of the appropriate repo skel somewhere, we need to know what the appropriate one is. When running regen-git for the first time (after this change), we can look at that repo skel checkout's origin and record that as the default parent.

Storing this in .vars.ini in a new section seems reasonable.

Start by tracing the first-run behavior of regen-git and how it reads or writes .vars.ini. Identify where the parent skel repository’s origin could be recorded as the default parent, then verify that the stored value is available for the proposed future skelup workflow.