fix: recognize BOM-prefixed skill frontmatter

[quuu]

Valid SKILL.md files saved with a UTF-8 BOM were skipped as missing name and description. Accept an optional leading BOM before the YAML delimiter; preserve raw content and the YAML-only frontmatter boundary.

Four regression cases fail before the fix. All 59 focused parser/discovery tests and TypeScript pass after it, and a manual skills add <local-dir> --list discovers the BOM-prefixed skill. Coverage includes LF/CRLF files and rejection of JavaScript frontmatter.

[vercel-agent-factory]

@quuu — bug-fix candidate.

• Fix: BOM-prefixed SKILL.md files were skipped as missing name and description.
• Tests: regression coverage in src/frontmatter.test.ts for LF/CRLF, BOM, and JS-frontmatter rejection.
• CI: in progress at time of review (head e0498a5f).

[6639835]

Reviewed the frontmatter matcher and parser/discovery tests at e0498a5. The optional U+FEFF remains anchored immediately before the YAML delimiter, preserving LF/CRLF and YAML-only behavior. Unmatched content remains unchanged; discovery retains rawContent. Tests cover BOM/no BOM, both newline styles, unsupported JavaScript headers, and text without frontmatter. No actionable issue found. Static review only; Vitest and the CLI were not run.