Update guidelines for code review process

Added a guideline to justify or remove unused/dead code.

Author: softhack007

.github/copilot-instructions.md

@@ -69,6 +69,7 @@ Main development branch: `mdev`
 - **Acknowledge good patterns** when you see them. Summarize good practices as part of your review - positive feedback always helps.
 - **Provide references** when making analyses or recommendations. Base them on the correct branch or PR.
 - **Look for user-visible "breaking" changes**. Ask for confirmation that these were made intentionally.
+- **Unused / dead code must be justified or removed**. This helps to keep the codebase clean, maintainable and readable.
 - **C++ formatting available**: `clang-format` is installed but not in CI
 - No automated linting is configured — match existing code style in files you edit. See `cpp.instructions.md` and `web.instructions.md` for language-specific conventions, and `cicd.instructions.md` for GitHub Actions workflows.