docs: frame branch->refs migration as read-routing, not mirroring

The git-refs backend doc originally presented "git-branch as a mirror of git-refs" as the safe rollout (a Parallel state) whose job was downgrade safety. That framing is wrong for this migration:

• Every reader — CLI, entire.io, and entire-api — already routes reads refs-first for both ID formats and falls back to the branch for the legacy format. So cross-format reads need no dual-write.
• An old client that cannot read the new ULID/refs format fails closed and gets an upgrade nudge via checkpoint_min_version, which is the intended forcing function, not a regression to paper over.
• The mirror also never delivered remote downgrade safety anyway: under a git-refs primary, pre-push returns after pushing refs and never pushes the mirror's v1 branch.

Rewrite the taxonomy note, config example, rollout table, and Migration section to state the real model (primary flip + read routing + version policy), drop the "v1 mirror push for downgrade safety" limitation, and reframe the ULID-on-branch guard: since git-branch is not a mirror of git-refs, a ULID reaching the branch write path is simply a bug to reject, not a topology-role-aware check. Mirroring remains documented as a general mechanism (e.g. non-git-backed targets), just not part of the branch->refs path.