Some improvements to loading the session with --prompt-cache (#1550)

Improvements to loading the session with `--prompt-cache` in the `main` example.

1. Fix an issue where the `--seed` parameter was ignored when loading a cached prompt.
2. When loading a cached prompt, you previously had to specify the saved prompt (or a prefix of it) again. This pull changes that behavior to default to the prompt that was cached if a prompt wasn't specified by the user.