Pin the last user prompt in the top bar (like Grok Build)

### Feature request

Keep my most recent prompt pinned in the top bar of the session, the way Grok Build does it.

**Problem**

During a long session the agent produces a lot of output (tool calls, logs, diffs). After scrolling for a while I lose sight of what I actually asked for. To re-read my own last instruction I have to scroll back up through hundreds of lines, and in the desktop app that scrollback can be very long.

**Proposal**

Show the latest user message pinned in the top bar / header area:

- Always visible while the agent is working, not affected by scroll position.
- One line, truncated with an ellipsis; expandable on click/hover for the full text.
- Updates on every new user message.
- Ideally the same for the CLI (a sticky line above the input) and the desktop app (Code tab header).

**Why it helps**

- Keeps the current goal in view so it is obvious when the agent drifts off task.
- Removes the scroll-hunting for "what did I ask again".
- Useful during long autonomous runs where the operator steps away and comes back.

Nice to have: a small toggle to pin an earlier prompt instead of the latest one.

No files or tests are named. Start by finding where the CLI session UI and the desktop app Code tab header are rendered, then trace where the latest user message is stored or displayed. Done means the latest prompt stays visible, truncates to one line with access to the full text, updates on each new user message, and does not move with scroll position.

索引モデルが issue の本文から書いたものです。