Add CHANGELOG entry for the phantom-merge fix (merge-verify sub-gate,
deterministic merge actor, post-deploy verification, kill-switch).
Addresses P0 blocker from reviewer (attempt 2/3): docs = golden source
per CLAUDE.md §2/§6 and AC-5.
Refs: ORCH-071
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>