chore: Add Copilot instructions

Author: nzakas

.github/copilot-instructions.md

@@ -0,0 +1,45 @@
+# Copilot Instructions for mentoss
+
+## Project structure
+
+- `src/` - Source code for the project.
+- `tests/` - Unit tests for the project.
+    - Test files are named to match the source files they test, with a `.test.js` suffix.
+- `docs/` - Documentation for the project.
+
+## Key Implementation Details
+
+- The `MockServer` class provides helpers for route matching, request/response tracing, and test assertions.
+- The `traceCalled(request)` method returns an object: `{ traces: Array<Trace>, matched: boolean }`.
+    - If a called route matches, it immediately returns `{ traces: [], matched: true }`.
+    - If no called route matches, it collects trace info from both called and unmatched routes and returns `{ traces, matched: false }`.
+- The `called(request)` method uses `traceCalled` and:
+    - Returns `true` if a called route matches.
+    - Returns `false` if no called route matches but traces exist.
+    - Throws an error with the message `"This request pattern doesn't match any registered routes."` if both `matched` is `false` and `traces` is an empty array.
+- The error message for unmatched patterns is standardized for consistency in code and tests.
+- Tests for `traceCalled` and `called` expect the above behaviors and error messages.
+
+## Documentation
+
+- Changes to the public API must be documented in the `docs/` directory.
+- Documentation should be clear and concise, explaining the purpose and usage of each public method.
+- Use examples to illustrate usage where appropriate.
+
+## Testing/Validation
+
+- Never remove failing tests.
+- Always add new tests for any new functionality or changes to existing functionality.
+- When modifying existing tests, ensure they still validate the intended behavior.
+- When adding new tests, ensure they cover all edge cases and expected behaviors.
+- Tests should be clear and concise, focusing on the specific functionality being tested.
+- Use descriptive names for test cases to indicate what functionality is being tested.
+- All tests must pass for changes to be considered complete.
+- Use `npm test` to run all tests and ensure they pass before submitting changes.
+- Use `npx mocha <filename>` to run a specific test file if needed.
+- Do not `cd` into the project directory; run commands from the current working directory.
+
+## General Guidance
+
+- When making changes to route matching, tracing, or test helpers, update both implementation and tests to keep them in sync.
+- Always keep error messages and return value structures consistent between code and tests.