Get Co Changes

Co-change pairs from git_metadata.co_change_partners_json.

With ?file=..., returns the partners that historically change with that file (one row per partner). Without, returns the top global pairs ranked by co_change_count, deduplicated as undirected pairs.

co_change_count is the number of shared commits, not the indexer's decay-weighted score. The client renders it as "x N", so a count is the only reading that makes the label true; the weighted score is what the coupling graph surfaces as strength.