fix(tdd): link testing anti-patterns reference

Fixes #1529.

skills/test-driven-development/SKILL.md

@@ -356,7 +356,7 @@ Never fix bugs without a test.
 
 ## Testing Anti-Patterns
 
-When adding mocks or test utilities, read @testing-anti-patterns.md to avoid common pitfalls:
+When adding mocks or test utilities, read [testing-anti-patterns.md](testing-anti-patterns.md) to avoid common pitfalls:
 - Testing mock behavior instead of real behavior
 - Adding test-only methods to production classes
 - Mocking without understanding dependencies