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/error-handling/SKILL.md

@@ -1,6 +1,6 @@
 ---
 name: error-handling
-description: Best practices for error handling across languages — exceptions, Result types, input validation, error boundaries, null safety, and observability. Use when designing error strategies, writing validation logic, handling API errors. Detects from code context and project files — not dependent on specific language keywords."
+description: MANDATORY — always active. Best practices for error handling across languages — exceptions, Result types, input validation, error boundaries, null safety, and observability. Use when designing error strategies, writing validation logic, handling API errors. Detects from code context and project files — not dependent on specific language keywords."
 ---
 
 # Error Handling