rewind picker: fall back to earlier prompt in multi-session checkpoints

When a multi-session condensed checkpoint's most-recent session has no prompt, the picker previously displayed an empty prompt even when an earlier session had a usable one. Walk sessionPrompts from latest to oldest and use the first non-empty value.

Adds a TDD test that writes two sessions of the same checkpoint where the latest carries no Prompts, and asserts the picker surfaces the earlier session's prompt.