doctor migrate-checkpoints: don't report an empty queue as "push disabled"

PushQueuedCheckpointRefs returned (0, nil) both when push_sessions is disabled and when the queue is empty, so the doctor command reported "Checkpoint pushing is disabled in settings" even when pushing was enabled and the refs had simply already been pushed — e.g. a concurrent git push flushed the just-migrated refs while the confirm prompt blocked.

Return an explicit pushDisabled flag alongside the pushed count so the two cases are distinguishable. The command now prints the disabled message only when pushing is actually disabled, and a distinct "no queued refs to push" message otherwise. Adds a push-disabled test and updates the existing entry-point tests for the new signature.