Cloud session bundling: HEAD-behind-remote is now its own verdict

The git divergence check used for cloud session bundling now tells head_behind_remote apart from head_not_at_remote using a merge-base exit code, and warns when ancestry can't be determined. When HEAD is merely behind the remote, the caller can now start bundling from the local HEAD commit instead of treating it as diverged. A new reason-phrase map covers bundling unavailability, the size limit, and lack of Windows pull support, and the message shown when a bundle fails to arm is now composed from that map rather than a single fixed sentence.