Add .github/copilot-instructions.md for repository context

[Copilot]

GitHub Copilot has no repository-specific context for this codebase, leading to suggestions that may not align with ComfyUI's node contract or the project's conventions.

Changes

• .github/copilot-instructions.md — New file providing Copilot with:
  • Annotated repo structure (key files and their roles)
  • ComfyUI node class pattern (INPUT_TYPES, RETURN_TYPES, FUNCTION, CATEGORY, TSC_ prefix convention)
  • Node registration pattern via NODE_CLASS_MAPPINGS and conditional try/except ImportError for optional features
  • Model caching architecture (loaded_objects, last_helds, globals_cleanup())
  • Frontend JS extension conventions (app.registerExtension, "efficiency.<name>" naming)
  • Critical development guardrails — node contract stability, cache preservation, output formatting helpers
  • Validation approach and publish workflow overview