test(review): pin the git-branch backend in enableReviewAgent

Defensive, matching sha256_repo_test.go: this file's helpers (readCheckpointSummary/readSessionMetadata) resolve checkpoint content from the v1 metadata branch. Today the tests pass either way — their TestEnv pre-writes .entire/settings.json, so enable --agent is not a first run and the git-refs default never fires — but the pin makes the backend assumption explicit instead of depending on that setup detail.