MR-3 — Sibling files stay sibling

When two files exist as a pair (wire-contract emitter + agent-emitter; forward-migration + rollback-migration; OSS-mode-config + cloud-mode-config), do not merge them.

Merging conflates concerns and re-introduces the very drift the pair was created to prevent. If a justification for merging exists, it goes in a decision doc before the merge — not after.

The implementer subagent has a refusal condition for this. Trust it.