Persist upstream Radar review artifacts

[yvette-carlisle]

Summary

• add source-backed upstream reviews for openai/codex PRs #27535, #27539, and #27936
• add upstream-impact classifications for all three reviewed PRs
• add Publisher social-candidate handoffs for #27539 and #27936 only

Validation

• cargo run -p decodex --bin decodex -- radar bundle validate artifacts/github/bundles/openai-codex-pr-27535.json
• cargo run -p decodex --bin decodex -- radar bundle validate artifacts/github/bundles/openai-codex-pr-27539.json
• cargo run -p decodex --bin decodex -- radar bundle validate artifacts/github/bundles/openai-codex-pr-27936.json
• jq empty on new review/impact/social-candidate artifacts
• cargo run -p decodex --bin decodex -- radar validate
• git diff --check origin/main...HEAD

No social_post/v1 records were written and nothing was published to X.