Persist Decodex X PR 27961 reservation

[yvette-carlisle]

Summary

• add the PR 27961 Decodex X publication reservation for the 2026-06-16 Asia/Shanghai cap day
• keep the reservation thin: lease, source refs, duplicate keys, owner metadata, and pre-compose evidence only

Validation

• cargo run -p decodex --bin decodex -- radar validate artifacts/github/social-candidates artifacts/social/x

Publication gate

• candidate: artifacts/github/social-candidates/openai-codex-pr-27961.json
• decision.worthiness: publish
• target account readback: Chrome active account showed Decodex @decodexspace before reservation
• duplicate gate: checked records/open PRs/profile/X search had no 27961 or remote-control policy duplicate
• media: no generated media planned; non-release operator-impact post remains valuable text-only with the GitHub PR card