fix(writing-skills): use markdown link for testing methodology reference

skills/writing-skills/SKILL.md

@@ -553,7 +553,7 @@ Run same scenarios WITH skill. Agent should now comply.
 
 Agent found new rationalization? Add explicit counter. Re-test until bulletproof.
 
-**Testing methodology:** See @testing-skills-with-subagents.md for the complete testing methodology:
+**Testing methodology:** See [testing-skills-with-subagents.md](testing-skills-with-subagents.md) for the complete testing methodology:
 - How to write pressure scenarios
 - Pressure types (time, sunk cost, authority, exhaustion)
 - Plugging holes systematically