Interactive queue manager — browse, inspect, and remove queued prompts with confirmation

### Describe the feature or problem you'd like to solve

Once prompts are queued, they become a black box. There is no way to see what's in the queue (full text), navigate between entries, or remove a specific one without cancelling everything. If you queue 4 prompts and realise the 2nd is wrong, you have no surgical option — you either let it run or lose all of them.

Related: #1683 (reorder) — this issue is specifically about **inspection and selective removal**.

### Proposed solution

Add an interactive TUI queue manager, accessible via a keyboard shortcut (e.g. `Ctrl+Q` or a `/queue` slash command) while prompts are queued. The panel would show all pending entries in a scrollable list and support:

- **↑ / ↓** — navigate between queued items
- **Enter / Space** — expand to see the full prompt text
- **Delete / Backspace** — remove the selected item, with a one-step inline confirmation:
```
Remove "Update the README with the new API docs"? (y/N)
```
- **Escape** — close the panel without changes

The queue badge/counter already visible in the UI (e.g. `[2 queued]`) could serve as the entry point — making it clickable or keyboard-focusable to open the manager.

### Example prompts or workflows

1. User queues 4 prompts. Realises #2 has a typo or is wrong. Opens queue manager, navigates to #2, removes it with confirmation — #3 and #4 slide up without disruption.
2. User queues a long prompt. Opens manager to read its full text before it runs to verify it's correct.
3. User queues 3 prompts, decides the last 2 are redundant after seeing the agent's output. Opens manager, removes both one at a time with confirmation each time.
4. Accidental Delete keypress in the manager is protected by the `(y/N)` confirmation — no prompt is lost silently.

### Additional context

- OS: Windows 11 Pro (Build 26200 / 24H2)
- Shell: PowerShell 7.5.4
- Copilot CLI: 0.0.420 (win32-x64)
- The existing alt-screen TUI (used for `/context`, checkpoints, etc.) is a natural host for this panel — same scrollable list pattern.