Context Compaction needs Guardrails

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

The copilot_instructions.md needs a mechanism for marking critical sections that must be reapplied after compaction.

### Proposed solution

Use code fence to mark sections of the copilot_instructions.md that must be reapplied after compaction.

### Example prompts or workflows

```critical
# ALWAYS DO THIS

xxxxxxx
```

### Additional context

I spend so much time nagging copilot to reapply copilot_instructions.md after compaction, and each request to do so uses premium requests.