docs(vrs): specify Codex native DING delivery

[myobie]

Summary

• define the deliver "app-server" Codex transport
• require a persisted runtime-scoped thread binding before delivery
• define idle turn/start, active regular turn/steer, and fail-closed hold states
• define typed userMessage acceptance and st2-owned duplicate control
• define bounded complete-body FIFO views without changing durable settlement
• keep the existing ding screen grammar unchanged

Dependency

This change is stacked on #229. Review this pull request as the Codex-specific addition to that shared DING contract.

Current protocol evidence

I verified Codex CLI 0.145.0 through its generated app-server v2 JSON Schema and one ephemeral live app-server thread.

• turn/start and turn/steer accept clientUserMessageId.
• turn/steer requires exact expectedTurnId.
• The idle turn/start response was followed by item/completed with item.type = "userMessage" and item.clientId equal to the supplied client ID.
• The same turn produced the expected observable model response.
• turn/steer while idle failed with no active turn to steer.
• The generated schema identifies review and manual compaction as non-steerable active turns.

The contract follows the current official Codex App Server manual. It also states that this is an experimental provider surface and must fail closed on an incompatible schema.

Validation

• codex app-server generate-json-schema on Codex CLI 0.145.0
• ephemeral live app-server wire observation
• git diff --check
• hosted Nix run 31502005706 succeeded at exact head 37a14da

Review

Please review the exact head 37a14da. This pull request remains draft and must not merge before Nathan reviews it.

[myobie]

Folded into the reviewable specification replacement #242 at exact head b5debb736b5fd416eac7294849ed816242bd5625. The replacement preserves this draft’s relevant history and has exact-head hosted Nix green. Closing this superseded draft; its branch is intentionally retained.