Codex Desktop: open Sites entries in the companion browser

Feature request

When a user clicks a site listed in the Sites section of Codex Desktop, open that site directly in the Codex companion browser.

Motivation

The Sites workflow should keep preview, iteration, and validation inside Codex. Requiring users to copy a preview URL or opening it outside the companion browser interrupts the workflow and makes local-site testing less convenient.

Expected behavior

• Clicking a site name, site card, or preview action opens its current preview or published URL in the Codex companion browser.
• Reuse an existing companion-browser tab for the same site when possible; otherwise open a new tab.
• Preserve the Codex-authenticated session and access to localhost previews.
• Support both localhost previews and deployed Sites URLs.
• Show a clear status/error when a site is still building, unavailable, or has no preview URL.
• Keep Open in external browser as an optional secondary action.

Acceptance criteria

1. A click on a valid Sites entry opens the site in the companion browser without manual URL copying.
2. Localhost and deployed site URLs both work.
3. Repeated clicks do not create unnecessary duplicate tabs for the same site.
4. Authentication/session state is preserved where applicable.
5. Unavailable or build-in-progress sites produce an actionable message.
6. Users can still explicitly choose an external browser.

This request is specifically for the Codex Desktop app's Sites section and companion-browser integration.