Remove unrelated .github files from SDK manager PR

These files were part of a separate copilot instructions PR (#285/#287).
Main now has its own copilot-instructions.md. Removing:
- CSharpExpert.agent.md (not in main)
- principal-software-engineer.agent.md (not in main)
- copilot-instructions.md changes (reverted to main)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: rmarinho