fix: preserve folded skill descriptions in s05

[Gujiassh]

Summary

• preserve folded YAML block-scalar values in SkillLoader._parse_frontmatter so SKILL metadata keeps multiline descriptions
• keep the parser dependency-free instead of relying on undeclared YAML packages
• add a focused regression test for description: > frontmatter

Testing

• python3 -m pytest tests/test_s05_skill_loading.py

Closes #79

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