Support adding skills from more github repos

[proschowsky]

I would love to use agr more, but often find that it cannot import skills due to the way remote repos are structured.

The user/name and user/repo/name handles works great for supported repos, but when skills are places in other folders, it would be great to be able to give the url to the folder instead.

Examples:

https://github.com/anthropics/claude-plugins-official/tree/main/plugins/skill-creator/skills/skill-creator
https://github.com/mizchi/skills/blob/main/tooling/justfile/