There is one potential issue with scanning the entire codebase IMHO - if the codebase is old enough, there can be old snippets of code that DON'T correspond to the current preferences anymore. So scanning the last 200 PRs and reading comments is a bit safer maybe.

This is what my vibe coded PoC did - it just used GitHub API to go through the last X merged PRs, see the diffs and all discussions and create a document based on that - the developer manifesto kind of, a set of preferences and rules for this dev team. Then, use this manifesto as context whenever reviewing new PRs