Update copilot-instructions.md

Author: siriak

.github/copilot-instructions.md

@@ -1,3 +1,5 @@
 When reviewing a pull request:
 - Verify that any added algorithms or data structures aren’t already implemented elsewhere in the repository (including under a different name).
 - Confirm that the proposed algorithm is a recognized computer-science algorithm, not a problem-specific adaptation of a general technique (e.g., tuned for LeetCode or other competitive-programming problems).
+- Check that the extension of all code file names is a lowercase r.
+- Check that DIRECTORY.md was updated correctly.