copilot: Tell the AI to be thorough

At least when doing a local review, asking it to be thorough results
in a lot more comments on coding style violation so let's add such an
instruction to the global instructions.

.github/copilot-instructions.md

@@ -21,6 +21,7 @@ Include any other files from the [documentation](../docs) in the context as need
 - Frame comments as questions
 - Always consider you may be wrong
 - Do not argue with contributors, assume they are right unless you are very confident in your deduction
+- Be extremely thorough. Every single separate coding style violation should be reported
 
 ## Testing Expectations