docs(skill): clarify unit test triggers

shared/skills/implementation/unit-test-write/SKILL.md

@@ -1,10 +1,19 @@
 ---
 name: unit-test-write
-description: Add focused unit tests for isolated behavior.
+description: Use when adding or changing tests for one isolated function, component, or decision. Use integration-test-write when behavior must cross an application or service boundary.
 ---
 
 # Unit Test Write
 
-Identify the behavior and important boundary cases, then add focused deterministic tests.
+Use when adding or updating tests for one isolated function, component, or decision.
 
-Prefer observable behavior over implementation details. Report the checks that were run.
+## Workflow
+
+1. Read the requirement and implementation. Identify the observable result that defines success and the boundary conditions most likely to fail.
+2. Follow the repository's existing test framework, naming, setup, and assertion style. Avoid new dependencies or fixtures when a small test can express the behavior.
+3. Cover the normal case and the smallest meaningful set of boundary or failure cases. Include a regression case for a reported defect.
+4. Assert public behavior and useful error outcomes rather than private call order, incidental wording, or internal data layout.
+5. Keep each test deterministic. Control time, randomness, filesystem state, and external services when they affect the result.
+6. Run the focused test and report its result. Broaden verification only when shared behavior or connected components are affected.
+
+Do not add tests that duplicate existing coverage or require production code to expose test-only hooks without a clear reason.