Deno Guidance

This is essential scaffolding for anyone working with Deno projects. It handles the foundational stuff like configuring deno.json, choosing between JSR and npm packages, and running CLI commands. The most valuable part is its aggressive stance against deprecated URL-based imports from deno.land/x, which older LLMs still default to. It prioritizes JSR packages over npm when available and includes clear guidance on the standard library migration to @std/ on JSR. Also covers the deno update versus deno upgrade distinction, which trips people up constantly. If you're building anything in Deno, load this first to avoid outdated patterns.