Add support for installing skills and agents from git repos with non-standard domain names (e.g. github enterprise)

[gtshepard]

Problem

Installing a skill from from git repos with non-standard domain names (e.g. github enterprise instances with a custom domain) currently requires a local clone and --provider local:

gitagent skills install --provider local <local-path>/<repo>/skills/<skill>

This clone step adds friction to development workflows and is inconsistent with gitagent run -r, which already accepts a repository URL, for example:

gitagent run -r https://<mygit>.com/<org>/<repo>

Solution

Allow skills (and agents) to be installed directly from a Git repository URL, similar to gitagent run -r.

Possible formats:

gitagent skills install --provider git https://<mygit>.com/<org>/<repo>/skills/<skill>

or

gitagent skills install -r https://<mygit>.com/<org>/<repo>/skills/<skill>