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_

No implementation files or tests are named. Start by locating how VS Code-selected files become @filename context in Copilot CLI, then compare the existing context display and command handling. Done means one selected file can be removed or deselected without losing the conversation history, with coverage for the chosen interaction.

Written by the indexing model from the issue text.