Add chat export support using existing Codex logs

What feature would you like to see?

Codex CLI already saves full chat logs in ~/.codex, but they’re stored as verbose JSON with a lot of internal data.
This proposal is to make those chats exportable in a readable format (Markdown or plain text) that includes only user and model messages.

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Why is this useful?

• Lets users export a finished chat and continue it in another session, keeping context across resets.
• Complements issue #5348 (Session Chaining) — not the preferred long-term solution, but a practical interim step that supports the manual workaround discussed in its comments.
• Uses existing saved data, requiring only a simple export layer.
• Improves usability and reproducibility for long-running projects.

Additional information

This would be a low-risk enhancement that only exposes readable exports of logs Codex already stores locally.