Enable excluding initial and subsequent reviewers for a PR when retrying review

it's straightforward for a PR to get stuck in a review loop as the author issues "review ... again" over and over, cycling between the same set of reviewers. it'd be useful to store an association of reviewers to PRs in memory, with a configurable TTL, so that if a reviewer has been recently assigned, they can be excluded for the next retry.

Start by locating the code that handles the "review ... again" retry and assigns pull request reviewers. Trace how reviewer assignments are selected, then add the in-memory PR association and configurable TTL so recently assigned reviewers are excluded on the next retry; done means repeated retries can avoid the same reviewers.

Written by the indexing model from the issue text.