chore(copilot): refine instructions for writing new tests

Author: travi

.github/copilot-instructions.md

@@ -30,9 +30,9 @@ npm test
 Follow this exact sequence for ALL code changes. Work in small increments — make one change at a time and validate before proceeding.
 
 1. **Research**: Search codebase for existing patterns, commands, utilities. Understand how similar functionality is implemented.
-2. **Write failing integration test**: Create test describing desired behavior (use Cucumber for integration tests)
+2. **Write failing integration test**: Create test (or modify existing test) describing desired behavior (use Cucumber for integration tests)
 3. **Verify failure**: Run `npm run test:integration` — confirm clear failure message
-4. **Write failing unit test**: Create test describing desired behavior
+4. **Write failing unit test**: Create test (or modify existing test) describing desired behavior
 5. **Verify failure**: Run `npm run test:unit:base` — confirm clear failure message
 6. **Implement minimal code**: Write just enough to pass
 7. **Verify pass**: Run `npm run test:unit:base` — confirm pass