Make File References Clickable in Copilot CLI Output

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

Enable clickable links for file references in Copilot CLI responses, allowing users to directly open files from highlighted references.

Copilot CLI already detects and highlights file references (e.g., @vscode/l10n, contextKeyExpr.test.ts, compile-production) in blue, implying interactivity. However:

These references are not clickable
Users must manually search or navigate to the file
This introduces friction, especially in large repositories

From the screenshot:

Multiple file/module references are clearly recognized and styled
Despite visual affordance, there is no direct navigation action
The experience is inconsistent with user expectations from IDEs and terminals with link support

### Proposed Solution
Extend file reference detection to support clickable links that open the referenced file.
**Supported interactions**

Click or Cmd/Ctrl + click → open in new tab

### Example prompts or workflows

_No response_

### Additional context

_No response_