feat: add skill-tester POWERFUL-tier skill
- Comprehensive skill validation, testing, and quality scoring framework
- skill_validator.py: validates structure, documentation, and compliance (700+ LOC)
- script_tester.py: tests syntax, functionality, and runtime behavior (800+ LOC)
- quality_scorer.py: multi-dimensional quality assessment with scoring (1100+ LOC)
- Complete reference documentation (structure spec, tier requirements, scoring rubric)
- Sample skill with assets and expected outputs for testing
- CI/CD integration examples and pre-commit hook support
- Zero external dependencies, dual output formats (JSON + human-readable)
- Self-testing capable meta-skill for quality assurance automation
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