Allow removing/canceling a queued message without interrupting the active run

Start by inspecting the Command Code TUI handling for queued messages and the Esc/Ctrl+C interrupt action. Define the queue-management behavior and verify that removing a queued message leaves the active agent run untouched; the issue does not name specific files or tests.

Feature Description

Add a way in the Command Code TUI to remove/cancel a queued user message without interrupting the currently active agent run.

Today, while Command Code is working, a new prompt can appear as Queued (1). If I change my mind and no longer want that queued instruction to run, there does not appear to be a non-destructive way to remove it. Esc / Ctrl+C interrupts the active run, which is much more disruptive than simply deleting the queued message.

This is especially noticeable during long-running tasks: the foreground agent may have been working for 10–20+ minutes, and cancelling one accidental or obsolete queued prompt should not require stopping that work.

Use Case

Example workflow:

1. Start a long-running task.
2. While it is still running, type a follow-up such as "check all sub-agent statuses".
3. The prompt is shown as Queued (1).
4. Realize that the follow-up is unnecessary or would be better asked later.
5. Want to remove only that queued message while leaving the active run completely untouched.

Current options are poor:

• Wait for the active run to finish and let the unwanted queued message execute.
• Press Esc / Ctrl+C, which interrupts the active run too.

Expected Behavior

Provide a non-destructive queue-management action, for example one or more of:

• Select/click a queued message and choose Remove from queue.
• A shortcut such as Alt+Backspace / Ctrl+Shift+Backspace for the most recently queued message.
• A small queue panel where pending messages can be viewed, reordered, edited, or deleted.
• A command such as /queue with remove, clear, or edit actions.

Important semantic requirement: removing a queued message must not interrupt, steer, cancel, or restart the active agent run.

Actual Behavior

The TUI shows that a message is queued (Queued (1)), but there is no obvious way to remove only that queued message. The available interrupt action (Esc / Ctrl+C) affects the active run instead of just the queued input.

Additional Context

Related to #627 ("Edit Queued Messages"), but this request is specifically about cancelling/removing queued messages without touching the active run. Editing support would be useful too, but deletion/unqueue is important on its own because it provides a safe way to undo accidental queued prompts.

Also related historically to #356, whose report explicitly mentioned being unable to unqueue a message, although that issue focused on queued-message behavior after /compact.

Environment where this was observed: Windows, Command Code TUI in Windows Terminal / PowerShell.

How important is this to you?

Important for my workflow