Write unit and integration tests that actually catch bugs — real assertions, real edge cases, real failure modes. Detects the project's framework (Jest, Vitest, Mocha, pytest, Go test) and matches existing conventions. Use when the user says "write tests for", "cover this with tests", "add unit tests", "I need tests for this function", or hands over an untested module. Refuses to write placeholder `expect(true).toBe(true)` tests.
