Include model name in `exec --json` JSONL output

When running codex exec --json, the JSONL events (thread.started, turn.started, item.completed, turn.completed) don't include the model name. The interactive TUI shows the model, but the programmatic output doesn't.

It would be helpful if thread.started or turn.completed included a model field:

{"type":"thread.started","thread_id":"...","model":"codex-mini-latest"}

Why: Programmatic consumers of exec --json (CI pipelines, Telegram bridges like Untether, dashboards) have no way to know which model was used. Claude Code and Gemini CLI both include model in their JSONL init events.