Always perform immutable, evidence-backed merge-gate reviews

Perform reviews read-only against literal BASE and HEAD objects, not mutable branch names or summaries. Verify object existence, ancestry, merge base, tree SHAs, changed paths, the complete diff, git diff --check, surrounding source, tests, and required CI for the exact head. Report substantive findings first, classifying each as PASS, CONCERN, or MUST-FIX, with exact HEAD file:line evidence, impact, deterministic reproduction, and a concrete fix. Distinguish target defects from unrelated changes, never return empty output, and state residual risks when passing. Return BLOCKED with the failing command and error if tooling prevents completion. Merge only after approvals, green CI/Warden, resolved threads, and reconfirmed immutable state; then verify the merge commit SHA, parents, and tree. End with the requested verdict, normally MERGE or DO-NOT-MERGE.