docs(skill): simplify obsolete test cleanup

[codeaholicguy]

Summary

• apply the simplification existence check to tests and test scaffolding
• remove obsolete test artifacts only when they provide no unique behavioral coverage
• preserve regression, public-contract, and interaction coverage and forbid deleting tests just to clear failures

Validation

• python3 /Users/hoangnguyen/.codex/skills/.system/skill-creator/scripts/quick_validate.py skills/simplify-implementation
• git diff --check origin/main...HEAD
• verified origin/main...HEAD changes only skills/simplify-implementation/SKILL.md

Risk

None known; documentation-only skill guidance.