Cas Submodule Support

cas — Repositories with submodules now clone correctly

Cloning a repository that contains git submodules through the Content Addressable Store failed while ingesting the repository:

git_cat_file: fatal: Not a valid object name <hash>

A submodule appears in the repository tree as a pointer to a commit in another repository, so the object behind it cannot be read from the repository being cloned.

The CAS now fetches each submodule from the URL registered in .gitmodules at its pinned commit and materializes its contents in place, including nested submodules. Relative submodule URLs (such as ../sibling.git) are resolved against the parent repository URL, matching git's behavior. Submodule contents are stored and deduplicated like any other content, so repeated clones reuse the cache.