Add an in-thread conversation outline (auto subheadings + quick jump) for each single thread

Title
Add an in-thread conversation outline (auto subheadings + quick jump) for each single thread

Background
Today, Codex only provides a thread list in the left panel, but no internal outline within a thread. In long conversations, users must scroll up repeatedly to find earlier content. This creates high search cost and memory burden, and significantly hurts long-session usability.

Requirements

1. Add a “conversation outline” sidebar inside the current thread (not a new thread list).
2. Automatically generate one short subheading for each user question.
3. Clicking a subheading should jump directly to the corresponding turn.
4. Keep the outline updated in real time as the conversation grows.
5. Allow manual renaming of subheadings when auto summaries are inaccurate.
6. Highlight the outline item that matches the current reading position.

Interaction Proposal

1. Keep the existing left thread list unchanged.
2. Add a new collapsible “Outline” panel in the thread detail view.
3. Suggested default title length: 8-20 words/chars, truncate when too long.
4. Show full title and timestamp on hover.
5. Support pin/star for key items to improve review workflow.

Clarification vs. Current Behavior
Current navigation covers project-level or folder-level thread listing.
This request is specifically for thread-internal navigation within one conversation thread.

Acceptance Criteria

1. In 100+ turn conversations, users can reach any historical turn within 3 clicks.
2. Auto subheading coverage is above 95%, with manual edit support.
3. Jump targets are accurate, with no obvious scrolling performance regression.
4. Average time to locate historical questions is significantly reduced in long sessions.