discourage agent from using any/unknown casts in instructions (microsoft#269122)

add rule about as any/unknown

Author: meganrogge

.github/copilot-instructions.md

@@ -133,3 +133,4 @@ function f(x: number, y: string): void { }
 - Look for existing test patterns before creating new structures
 - Use `describe` and `test` consistently with existing patterns
 - If you create any temporary new files, scripts, or helper files for iteration, clean up these files by removing them at the end of the task
+- Do not use `any` or `unknown` as the type for variables, parameters, or return values unless absolutely necessary. If they need type annotations, they should have proper types or interfaces defined.