Add timestamp to exec JSON stream events

[hubix2000]

What

• Add a timestamp field to every exec JSONL event line (session creation time).
• Document the timestamp in codex exec --json output.

Why

• Needed to synchronize sessions later by querying JSONL updates since a recorded timestamp.

How

• Capture the session timestamp from SessionConfigured and inject it into each JSONL event line.
• Update exec JSON docs with the new field.

Refs #8620

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[hubix2000]

I have read the CLA Document and I hereby sign the CLA

[etraut-openai]

We've updated our contribution guidelines to clarify that we're currently accepting contributions for bugs and security fixes, but we're not generally accepting new features at this time. We need to make sure that all new features compose well with both existing and upcoming features and fit into our roadmap. If you would like to propose a new feature, please file or upvote an enhancement request in the issue tracker. We will generally prioritize new features based on community feedback.