rewind picker reads prompts via topology

Switch the picker's metadata tree read from the v1 branch to checkpoint.ResolveCommittedRefs(ctx).Read so v1.1 mode reads prompts from the mirror, matching the rewind execution path and 'entire explain'.

Adds a TDD test that places checkpoint metadata only on the mirror ref (v1 re-pointed to the initial commit) and asserts the picker still finds the prompt.