Get Commit Evolution

The repo's development "story arc" — commit-category mix over time.

Each commit was classified into exactly one category (feature / fix / refactor / docs / test / deps / chore / other) at index time and stamped onto its git_commits.json row. We bucket by month (or week on short-history repos) and stack the categories so the UI can show how the development emphasis shifts as the repo matures.

Declared before /commits/{sha} so the literal path wins over the SHA wildcard.