Support @ file includes in instruction files (like Claude Code)

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

Support @ inclusion in copilot-instructions.md

### Proposed solution

Copilot CLI should support @ file references inside instruction files (e.g., .github/copilot-instructions.md) to include content from other files,
similar to how Claude Code handles includes in its instruction files.

Currently, @ references only work in chat input, not in instruction files. This forces users to either duplicate content across multiple instruction files or use
symlinks/build steps as workarounds.

Use case: A repo with a shared instructions.md that should be included by both .github/copilot-instructions.md (for Copilot) and .claude/instructions.md (for Claude
Code). Without includes, maintaining consistency requires manual duplication or external tooling.

### Example prompts or workflows

_No response_

### Additional context

_No response_