feat(mirror): offer a cluster picker in the interactive one-shot create

When repo mirror create <github-url> is run on a terminal without a [cluster-host], fetch the control plane's cluster catalog and offer it as a single-select — pre-selecting the caller's jurisdiction default, and skipping the prompt entirely when only one cluster exists (the same prompt-only-when-there-is-a-choice shape as repo clone). Non-interactive invocations keep the fixed defaultClusterHost so scripts stay stable and offline-resolvable.