Allow users to cancel or remove enqueued messages before they are executed

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

There is no way to cancel or remove a message or slash command that has been queued (via `Ctrl+Q` / `Ctrl+Enter`) while the agent is busy or during `/compact`. Once enqueued, messages are automatically processed in order when the agent finishes — even if the user has changed their mind or made a mistake.

### Proposed solution

Add a mechanism to view and selectively cancel queued messages before they are executed. This could take the form of:
- A `/queue` command to list pending messages, with `/queue cancel ` to remove a specific entry.
- An interactive UI overlay (e.g., triggered by a keybinding) showing the queue with the ability to navigate and delete items.
- A keybinding (e.g., `Ctrl+Shift+Q`) to clear the entire queue at once.

This would benefit Copilot CLI users by:
- Preventing accidental or unwanted actions from queued messages the user no longer intends to send.
- Complementing existing cancellation behavior (`Ctrl+C` for active operations, `Escape` for specific commands like `/compact`) with control over *pending* operations.
- Improving confidence in the queueing workflow, encouraging users to queue more freely knowing they can course-correct.

### Example prompts or workflows

1. **Accidental queue during compaction** — User sends a message during `/compact` (auto-queued per v0.0.389), then realizes it was wrong. They want to remove it before compaction finishes and the message is sent to the model.
2. **Multiple queued commands** — User queues several slash commands with `Ctrl+Q` while the agent is working, then decides the third one is no longer needed. They run `/queue` to see the list and `/queue cancel 3` to drop it.
3. **Clear entire queue** — User has queued several messages but the agent's response changes their plan entirely. They press `Ctrl+Shift+Q` (or run `/queue clear`) to discard all pending messages and start fresh.
4. **Review before execution** — User wants to double-check what's queued before the agent finishes. They press a keybinding to see a transient overlay of queued items, then dismiss it or remove specific entries.

### Additional context

- Current queueing was introduced in v0.0.394 (`Ctrl+D`) and the keybinding was later changed to `Ctrl+Q` / `Ctrl+Enter` in v0.0.407.
- Auto-queueing during `/compact` was added in v0.0.389.
- Existing cancel mechanisms (`Ctrl+C`, `Escape`) only affect the *currently running* operation, not messages waiting in the queue.