Make default cluster jurisdiction-aware

is_default is per-jurisdiction (each of au/eu/us has its own default), so the single hardcoded "first default" was wrong. Resolve the caller's jurisdiction from /me and:

• one-shot repo mirror create <url> (no cluster-host): pick the is_default cluster for that jurisdiction;
• wizard: still list every cluster (so you can mirror into any), but pre-select only your jurisdiction's default instead of all three.

Errors asking for an explicit [cluster-host] when the jurisdiction is unknown or has no default. Surfaces the jurisdiction in the wizard's "Signed in as" line.