Agent Window discovers external Codex session but does not load its existing conversation history

### Does this issue occur when all extensions are disabled?

Not tested. This report concerns VS Code's Agent Host integration with an external Codex session. No third-party extension is required for the repro.

- VS Code Version: [paste from Help: About]
- OS Version: macOS 26.6.2
- ChatGPT for macOS: 26.901.22334 (build 7746)

### Steps to Reproduce

1. In ChatGPT for macOS, create or continue a local Codex session in a project and exchange several messages.
2. Quit ChatGPT for macOS, so no other client owns the session writer lock.
3. In VS Code, enable Codex Agent Host and open the Agents Window.
4. In the Sessions filter, set `External` to `Last 30 Days` or `All`.
5. Open the discovered external Codex session.

### Expected

The Agent Window should show the existing conversation history and allow the external Codex session to be continued.

### Actual

The session is discovered with the correct Codex provider, title/workspace, and recency, but its prior conversation history is not rendered in the Agent Window.

The underlying Codex session transcript exists locally. This is not a missing-session or writer-lock problem: the repro is performed after ChatGPT for macOS is closed.

### Additional observations

- With ChatGPT for macOS running, opening the same session can correctly fail with an active-writer conflict. That is a separate behavior and not the primary issue in this report.
- The regular Codex VS Code extension can subscribe to and display live state from the ChatGPT macOS session without a lock conflict.
- This suggests the failure is specific to loading/hydrating persisted history for a discovered external Codex session in Agent Window.

### Related

This appears to violate the external-session workflow covered by #331122, which expects external Codex sessions opened in the Agents Window to have their conversation represented sensibly.

I can provide sanitized logs and a screen recording privately if needed.