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

[dorlugasigal]

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 <n> 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.