[codex] Require structured headless review results

[romgenie]

What changed

Adds headless contract v2 for issue #119.

• Bumps the headless contract schemas and fixtures to version 2.
• Requires a structured review object in runtime results.
• Adds typed adapter-side review result models for mode, evidence status, reviewed files, findings, tests run, limitations, and no-findings rationale.
• Updates worker/API tests that deserialize or construct SessionResult.

Why

Dogfooding showed that hosted review evidence needs to be preserved as structured data. The adapter can prove a PR checkout and file list were supplied, but contract v1 had no required field for that proof in result.json.

Validation

• cargo test -p coven-github-worker contract
• cargo test -p coven-github-api
• cargo test -p coven-github-worker
• cargo test