Add manual bookmarks for important sections within a thread

What variant of Codex are you using?

App (macOS)

What feature would you like to see?

Codex threads should support user-created bookmarks: lightweight saved points inside a conversation that act like named return locations.

The goal is not a full table of contents or automatic thread outline. Instead, this would let users explicitly mark moments they know they will need again, such as:

• the original ask
• an approved plan
• a key command output
• a design decision
• a handoff summary
• the point where a bug was identified

One possible interaction:

• Hover a message or section and choose Bookmark icon beside the current Copy icon.
• Optionally give the bookmark a short name.
• Show saved bookmarks in the existing right-side panel area, alongside Outputs and Sources.
• Selecting a bookmark scrolls the thread back to that saved location.
• Bookmarks remain attached to the thread across app restarts.

This would make bookmarks more like persistent conversation anchors than search results or generated navigation. It would be especially useful when working through long tasks where a few specific moments matter more than the overall thread structure.

<img width="1643" height="957" alt="Image" src="https://github.com/user-attachments/assets/08d25db3-eebe-41e4-9f3b-1b17f2d81a57" />

<img width="2168" height="725" alt="Image" src="https://github.com/user-attachments/assets/6fad6175-e659-4638-aa6b-70b191e230e2" />

Related issue: #22627 - this request is unique in that it is user-created bookmarks but could exist alongside a generated ToC.

Additional information

_No response_