docs: record workspace membership gate merge

[yha9806]

Summary

• Record yha9806/vulca-platform PR Fix real-brief proposal rubric seeding #39 / merge dff2331f95161ec909a07b76ef7e94ae7def3cfe in the protected review context vault.
• Update Workspace durable status, production persistence spec, release readiness gate, M5 closeout, source index, and manifest.
• Preserve the boundary that Fix real-brief proposal rubric seeding #39 is a compatibility-route save/clear membership gate, not full user/JWT auth, read authorization, membership admin APIs/UI, typed Workspace aggregates, operation-specific writes, ingress proof, multi-instance readiness, or product-level R5.

Test Plan

• python3 docs/review-context/gates/validate_review_context.py
• python3 -m json.tool docs/review-context/MANIFEST.json >/dev/null && python3 -m json.tool docs/review-context/release-readiness/m5-closeout-summary.json >/dev/null
• git diff --check
• git diff --cached | gemini-agent diff-review --stdin