repo mirror detach: name the target with --into /et/<project>/<repo>

Replace --project and --name with one native ref, so the command reads entire repo mirror detach /gh/<owner>/<repo> --into /et/<project>/<repo>: the project half is resolved by name (it is a path segment, never a ULID), and the repo half is the native name the detach is asked to use. A wrong-way or malformed --into is refused before any request.