docs(copilot): add copilot review instructions for PRs (#19064)

cc @overlookmotel it should hopefully prevent copilot from reporting
errors on stuff like unused variables in test case files

.github/copilot-instructions.md

@@ -0,0 +1,11 @@
+# Copilot Review Instructions
+
+When reviewing pull requests in this repository:
+
+- Treat files inside any `fixtures` directories as test data, not production code.
+- Fixture files may intentionally contain buggy, unsafe, or syntactically invalid code to validate parser/linter/transformer behavior.
+- Do not report normal code-quality or correctness issues for fixture files.
+- You may report a fixture issue only when the test data appears incorrect or mislabeled:
+  - The folder or filename does not match what the test claims to cover.
+  - The fixture content does not actually test the described case.
+  - The fixture appears accidentally broken rather than intentionally crafted.