Codex campaign adapter: compile an API-compatible structured-output schema

[jeffhuber]

Problem

The maintained Codex release-campaign adapter passes the full closed adoption-result JSON Schema to Codex CLI 0.147.0. The OpenAI structured-output endpoint rejects the conditional if keyword inside steps.items, so the adapter exits before Codex can qualify the release.

This was reproduced during the v1.0.9 six-provider campaign. Isolated-home authentication and the CLI startup handshake both passed. Running the same qualification without the unsupported schema hint produced a result that passed Code Mower's closed validator.

Ask

• Compile a provider/API-compatible response-format schema for Codex rather than passing unsupported validation keywords.
• Keep the full closed local validator authoritative, including the conditional failure-reason rules.
• Add an offline fixture/test that proves the schema sent to Codex contains only supported structured-output keywords.
• Add a focused adapter test that a valid Codex result still passes and an invalid conditional result is rejected locally.
• Keep provider output transient and preserve the metadata-only privacy boundary.

Acceptance Criteria

• A maintained Codex campaign qualification reaches the provider on Codex CLI 0.147.0.
• The response-format schema no longer contains unsupported if, then, or else keywords.
• Local closed validation remains behaviorally unchanged.
• Tests and one documentation line cover the compatibility split.

Found while closing #729 after #734.