Add .github/copilot-instructions.md for Copilot context
Add repository instructions for GitHub Copilot sessions covering:
- Build system phases (update/build/test) and key flags
- High-level architecture (graph, optimizer, session, providers)
- C++ conventions (error macros, container types, span/string_view)
- Python conventions and testing patterns
- C API guidelines and PR expectations
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
