fix(cli): scope skill metadata to frontmatter

[WNZhao]

Summary

• read inferred skill metadata only from a leading YAML frontmatter block
• support BOM and LF/CRLF frontmatter boundaries
• add a regression proving Markdown body examples cannot overwrite generated manifest metadata

Bug

sh1pt skills new searched the entire SKILL.md for name: and description: lines. A documentation example in ordinary Markdown body text was therefore treated as manifest metadata.

Before the fix, the regression generated body-example; after the fix it correctly falls back to the containing directory plain-skill.

Validation

• pnpm exec vitest run packages/cli/src/commands/skills.test.ts --reporter=dot (18/18)
• git diff --check