Copilot Code Review incorrectly complains about "PROTOTYPE" comments in feature branches

"Avoid adding new PROTOTYPE tracking comments here. If follow-up work is required, please track it with an issue (and link it) or remove the comment; otherwise it becomes hard to know whether this is intentional technical debt or accidental." ([comment](https://github.com/dotnet/roslyn/pull/83553#discussion_r3184123929))

PROTOTYPE comments are allowed in feature branches. But they are disallowed in `main` branch. This is enforced by CI.
Let's update the copilot instructions or code review skill to avoid this incorrect warning.