XY-896: Dogfood standard Decodex Review checkpoint on a tiny docs lane

[yvette-carlisle]

Summary

• Clarify that valid standard-review dogfood evidence requires runtime issue_review_checkpoint evidence.
• State that manual PR review comments or local self-check notes do not replace a non-empty review_checkpoints array from decodex evidence.
• Note that release readiness remains blocked when the runtime checkpoint is absent, even if the implementation PR lands.

Validation

• cargo make fmt
• cargo make lint-fix
• cargo make test: 1035 passed, 1 skipped
• git diff --check

Review Evidence

• Clean independent fresh-context Decodex Review checkpoint recorded for c390fc2.

Linear: XY-896