auth: remove cluster-binding model and command surface

Removes the cluster→context binding now that account selection is recomputed per operation:

• contexts: drop File.Resolve and BindCluster. The ClusterContexts field is kept but inert (deprecated) so an existing contexts.json round-trips without data loss during the transition; Delete still prunes it and a full logout still clears it.
• auth: drop ClusterBindings/UnbindCluster, the entire auth unbind command, and the "Cluster bindings" section of entire auth contexts.

Tests updated accordingly; RemoveAllContexts still clears the legacy field (seeded directly now that BindCluster is gone).