fix(review): surface codex's human error, not the raw JSON blob

When codex fails, it wraps the upstream API error as a JSON string in its envelope message, so a failed reviewer row showed the whole blob (e.g. codex: {"type":"error","status":400,"error":{"message": "The 'gpt-5.6-sol' model requires a newer version of Codex..."}}) — the actual reason buried inside. cleanCodexFailureMessage unwraps it to the .error.message text so the row reads "codex: The 'gpt-5.6-sol' model requires a newer version of Codex...". Plain (non-JSON) messages and JSON without a message pass through unchanged; unwrapping is bounded against multiple nesting. Found while diagnosing a live codex failure (model/CLI-version mismatch) that the raw blob made hard to read.