refactor(trail): trim the bare-number guard to what the server cannot do

The server's strict branch check already refuses a repository processed in another cell, and rejects values it cannot accept without writing, so the CLI no longer repeats those checks. The note for a server that does not confirm the check is dropped: this change ships only after every cell runs the check. Kept: sending the branch, the detached-HEAD and unresolved-repository refusals, and the wording of both server refusals.