ci: add gh-pr-review merge gate

[DankerMu]

Adds a GitHub Actions gate that maintains a PR status context gh-pr-review based on a PR comment marker.

This allows branch protection to enforce the local review workflow (run $gh-pr-review, then paste its result comment).

Marker format (single line):

No functional code changes.

[DankerMu]

Review: gh-pr-review

• ✅ Scope: adds .github/workflows/gh-pr-review-gate.yml only.
• ✅ Security: pull_request_target workflow does not checkout or execute PR code; only writes commit status; marker accepted only from maintainers (OWNER|MEMBER|COLLABORATOR).
• ✅ Merge gate: status flips to success only when marker SHA matches PR head SHA and score/decision pass.

LGTM.