Add _check_skill_completeness() method to quality checker that validates:
- Prerequisites/verification sections (helps Claude check conditions first)
- Error handling/troubleshooting guidance (common issues and solutions)
- Workflow steps (sequential instructions using first/then/next/finally)
This addresses G2.3 and G2.4 from the roadmap:
- G2.3: Add readability scoring (via workflow step detection)
- G2.4: Add completeness checker
New checks use info-level messages (not warnings) to avoid affecting
quality scores for existing skills while still providing helpful guidance.
Includes 4 new unit tests for completeness checks.
Contributed by the AI Writing Guide project.
