Create instructions file for Copilot

.github/instructions/test-reporter.instructions.md

@@ -0,0 +1,15 @@
+---
+applyTo: '**'
+---
+
+Instructions:
+
+  - Ensure Node.js 20 is installed and used for all development and CI tasks.
+  - Do not update the Node.js runtime to versions newer than 20.
+  - Install all dependencies using `npm install`.
+  - Use TypeScript for all source code; ensure `tsconfig.json` is present and configured.
+  - Use `npm test` to run tests; ensure all tests pass before committing changes.
+  - Validate changes by running `npm run all` before submitting pull requests or merging.
+  - This project is a GitHub Actions action; ensure compatibility with GitHub Actions workflows.
+  - After validating changes, create a new commit with the `dist/` directory included.
+  - Document any changes to the action's inputs, outputs, or behavior in the README.