[github][copilot] add an instructions.md file

Unclear if this is doing any good, but the copilot agent does seem to
read it in.

.github/copilot-instructions.md

@@ -0,0 +1,14 @@
+# LK Kernel
+LK is an open source kernel intended to primarily run in supervisor mode on a variety
+of 32bit and 64bit architectures.
+
+It is designed to be small, fast, and efficient, with a focus on real-time performance.
+It is primarily written in C and assembly, with some parts in C++. The C++ parts mostly
+do not use the standard C++ library, and no exceptions.
+
+It is used in a variety of embedded and real-time systems, including automotive, industrial,
+and consumer electronics. It is also used in some academic and research projects.
+
+# Code Style
+4 space indentation, no tabs, or trailing whitespace.
+Header guards are implemented with #pragma once.