fix(skills): normalize Windows line endings

[dajiaohuang]

Summary

• normalize skill Markdown line endings before standalone validation
• normalize runtime skill content before frontmatter and section parsing

This keeps CRLF Windows checkouts equivalent to LF checkouts for validation, listing, and skill metadata extraction.

Fixes #47.

Validation

• npm run build
• npm run validate:skills
• focused CRLF copy check: standalone validator reports 13 skills; runtime validator reports 13 skills and 0 errors
• git diff --check