Add config.toml / .roborev.toml options to configure inclusion of previous review findings

[wesm]

By default, we include previous review findings from a branch in the review prompt, which was done in the hopes that it would positively influence the reviewer to focus on things that have not been already called out in previous reviews (anecdotally, this seems to be working well in codex, claude, and gemini, but I have not run systematic evals).

A few things to do:

1. disable this logic entirely
2. configure the lookback (current I think it's 5 review lookback, but maybe someone wants 2 or 10)