fix: make native review JSON contract explicit

[fscfede-beep]

Summary

Fixes #679.

The built-in review path intentionally returns prose rather than schema-shaped findings, but its --json payload previously omitted result, rawOutput, and parseError entirely. Consumers could not distinguish "no structured output exists" from a parse failure.

This makes the contract explicit without heuristically parsing reviewer prose:

• result: null
• rawOutput: the native review text
• parseError: an actionable explanation that built-in review returns prose and adversarial-review should be used for structured output
• reasoningSummary: exposed consistently with the structured review path

Human-readable review output is unchanged.

Validation

Windows 11 / Node 26.3.1:

• test-first reproduction failed on main because payload.result was undefined
• native JSON contract + existing review regressions: 3 passed, 0 failed
• node --check plugins/codex/scripts/codex-companion.mjs
• git diff --check

[sylvesterkaczmarek]

Making the JSON contract explicit is preferable to heuristic prose parsing. result: null distinguishes “no structured result exists” from a successful empty findings set, while rawOutput preserves the native reviewer output and parseError tells structured consumers which command owns that contract. Mirroring reasoningSummary at the same level also keeps the native and adversarial paths easier to consume consistently.