test+docs: pin the settings-driven git-refs default end to end

Review findings on PR #1789:

• TestEnableDefault_GitRefsRoundTrip exercises the shipped default without the env override every other backend-aware suite uses: enable writes the git-refs primary into settings, and that block alone drives condensation onto per-checkpoint refs.
• Architecture docs no longer call git-branch 'the default' — it is the config-less fallback; new setups get an explicit git-refs primary.