fix: add MANDATORY prefix to all 8 core skill descriptions

- Add MANDATORY — always active to engineering-principles, testing,
  error-handling, security, git-workflow, api-design
- Update hook output with explicit EXTREMELY_IMPORTANT instruction
- Skills now explicitly state they apply to every session

skills/testing/SKILL.md

@@ -1,6 +1,6 @@
 ---
 name: testing
-description: Best practices for software testing — TDD, test pyramid, F.I.R.S.T. principles, mocking strategies, and test organization. Use when writing tests, designing test strategy, refactoring under test. Detects from code context and project files — not dependent on specific language keywords."
+description: MANDATORY — always active. Best practices for software testing — TDD, test pyramid, F.I.R.S.T. principles, mocking strategies, and test organization. Use when writing tests, designing test strategy, refactoring under test. Detects from code context and project files — not dependent on specific language keywords."
 ---
 
 # Testing Best Practices