Feature Request: Add ability to remove individual file references from context

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

Currently, when files are opened/selected in VS Code, they automatically appear as context in Copilot CLI (shown as @filename). However, there's no way to remove individual files from context

### Proposed solution

Requested feature:
- Add a close/remove button (X) next to each @filename tag in CLI
- OR add a command like /remove-context or /deselect
- OR add ability to click the @filename tag to toggle it off

Use case:
When working on a project, I may accidentally select unrelated files in
VS Code, which then consume context memory unnecessarily. I want to remove
these without losing my entire conversation history.

This would provide feature parity with the VS Code extension and give users
better control over their context window usage.

### Example prompts or workflows

_No response_

### Additional context

_No response_