Add comprehensive coding guidelines for GitHub Copilot to improve
quality and consistency of AI-generated code. Instructions cover C++
and Python development with language-specific best practices, build
system usage, and testing workflows.
Following GitHub Copilot's standard layout with general instructions
in .github/copilot-instructions.md and language-specific files in
.github/instructions/ directory using *.instructions.md naming.
No backport: This change is only for developers in master, so it doesn't
need to be backported.
Closesscylladb/scylladb#25374
902d70d6b2