Add `entire repo mirror create` onboarding wizard

Run entire repo mirror create with no args to onboard repos interactively: verifies auth, multi-select repos to mirror, multi-select regions, then creates every (repo, region) mirror in parallel and prints the clone URLs. The positional create <github-url> [cluster-host] form is unchanged.

Regions come from entire-core's GET /api/v1/clusters (ListClusters). The endpoint isn't deployed yet, so the /clusters spec fragment is hand-authored into core.openapi.json and the ogen client regenerated; a real spec refresh is idempotent.