[.github] Update copilot instructions. (#9837)

Copilot has a habit of making changes you didn't ask for.

Add some instructions to make it only do changes if asked,
in as the "Nullable Reference Types" section.

.github/copilot-instructions.md

@@ -8,6 +8,8 @@ This is the main branch targeting .NET 10.
 
 When opting C# code into nullable reference types:
 
+* Only make the following changes when asked to do so.
+
 * Add `#nullable enable` at the top of the file.
 
 * Don't *ever* use `!` to handle `null`!