ci: narrow spec instruction paths for Copilot review

Agent-Logs-Url: https://github.com/mongodb/node-mongodb-native/sessions/cb968079-3e20-4226-8b7a-bdf588ae854c

Co-authored-by: dariakp <[email protected]>

Author: Copilot

.github/instructions/spec-tests.instructions.md

@@ -1,5 +1,8 @@
 ---
-applyTo: "test/spec/**"
+applyTo:
+  - "test/spec/**/*.json"
+  - "test/spec/**/*.yml"
+  - "test/spec/**/README.md"
 ---
 These are unified spec test files copied from the mongodb/specifications repo.
-Do not suggest changes to these files. They must match the upstream spec exactly.
+Do not suggest changes to these files. They must match the upstream spec exactly.