[codex] Add Workspace production persistence spec

[yha9806]

Summary

• Add 15-workspace-production-persistence-spec.md as the protected product design for Workspace production persistence.
• Define product objects, authorization roles, database-backed storage requirements, conflict handling, release-gate safety, API migration, and acceptance gates.
• Wire the spec into the vault README, manifest, validator, source index, Workspace product model, M5 closeout, and validator tests.

Boundary

• This is a product spec and does not implement production persistence in vulca-platform.
• Product-level R5 remains blocked until implementation evidence and a human release owner decision exist.

Validation

• git diff --check --staged
• python3 docs/review-context/gates/validate_review_context.py
• python3 -m pytest tests/test_review_context_vault.py -q (11 passed)
• git diff --staged | gemini-agent diff-review --stdin (pass; no unsafe claims)