Describe the feature or problem you'd like to solve

There is no way to give system level instructions to Copilot-CLI outside of repo specific instruction files.

Proposed solution

Add a--system-prompt parameter for copilot-cli to pass system level instruction directly on top of the repo level instruction files.

Example prompts or workflows

copilot -p "make so cool stuff" --system-prompt "extra instruction on making cool stuff"

Additional context

No response