Fix Zen free review JSON output contract #648

Problem

The public-repository OpenCode fallback added in #646 reaches opencode-free/north-mini-code-free, but the model can return prose or nested control data. The strict review normalizer correctly rejects it, leaving reviews unpublished.

Acceptance criteria

• Request JSON-object responses from the Zen free model through its existing OpenAI-compatible model options.
• Preserve the strict downstream control normalizer and public-repository-only privacy gate.
• Cover the generated review configuration with a regression test.
• Prove the option with a real anonymous OpenCode 1.17.13 North invocation.

Unblocks ContextualWisdomLab/saju-caldav#14.