Uploads to cloud sessions refuse repositories with alternate ref storage, with clearer refusals

What

Before Claude Code uploads your working folder to a cloud session, it checks that the git repository looks safe to send. Git is the version-control tool that tracks changes to your files. This check is now stricter and explains itself more precisely.

• A repository that uses a different way of storing references (git's extensions.refStorage, for example reftable), or an environment where GIT_REFERENCE_BACKEND is set, is now treated as tampered with. The upload is refused with the new ref_storage message, "Nothing was uploaded." An environment variable is a setting passed to a program from outside it.
• A linked worktree, a second working folder sharing one repository, is refused if its shared git folder is not the one expected.
• The git folders are now held open while they are checked, and their identities are compared, instead of a simple yes or no check.
• Refusals now carry named reasons such as git_dir_tampered, work_tree_replaced and git_did_not_run_from_outside.
• Git's own error text is no longer shown, "because it can quote private files".
• The work_tree message used to say to check git status here. It now says to check that you can still open this folder.
• The included_config and borrowed_objects messages are shorter or handed off elsewhere.

Why

If your repository uses reftable storage, or you set GIT_REFERENCE_BACKEND, uploads to cloud sessions will now be refused. You get a clear message instead of an upload that may not reflect your files correctly.