Blocker: Compact can be triggered accidentally with no confirmation or Undo

### Describe the bug

The **Compact** action in the VS Code Copilot CLI experience can be activated accidentally and immediately rewrites the active conversation context without:

- a confirmation dialog,
- a clear warning that the operation cannot be reversed,
- an Undo action, or
- a supported way to restore the original conversation into the active UI/context.

This is a blocker-level data-loss UX issue. Although an underlying session record/checkpoint may remain on disk, that is not equivalent to preserving the user's active conversation. After accidental activation, the user cannot restore the session to its prior state or see the original conversation as it appeared before compaction.

### Affected version

- VS Code: 1.131.0 (Windows x64)
- Copilot CLI runtime integrated into VS Code; standalone `copilot` executable is not available in PATH
- Observed August 3, 2026

### Steps to reproduce

1. Open a long-running Copilot CLI session in VS Code.
2. Accidentally activate **Compact**.
3. Observe that compaction begins immediately without confirmation.
4. After it completes, attempt to undo or restore the original active conversation.
5. Observe that no Undo or restoration path exists.

### Actual behavior

A single accidental activation permanently replaces a large active conversation with a generated summary. In the observed session, compaction removed approximately 133,000 tokens from active context. The session checkpoint/log remained, but the original visible/active conversation could not be restored.

### Expected behavior

Before Compact runs, show a blocking confirmation that clearly states what will change and whether it can be undone. At minimum:

1. Require explicit confirmation for manual compaction.
2. Clearly state that the current active context will be replaced by a summary.
3. Provide an immediate Undo/Restore action backed by the retained session record.
4. Preserve access to the original visible transcript.
5. Consider moving Compact away from controls that are easy to trigger accidentally.

Please treat this as a release-blocking UX/data-preservation defect rather than a normal enhancement request. Irreversible context replacement must not occur from one accidental click.

### Additional context

No private session content or source code is included in this report.

Start by reproducing the Compact action in the VS Code Copilot CLI experience and trace the manual compaction entry point. Review how the session checkpoint/log is retained and how the active conversation is replaced. Done means manual compaction requires confirmation and provides a supported Undo or restore path while preserving the original transcript.

Written by the indexing model from the issue text.