fix(skills): preserve frontmatter fence variants

[imshunsuke]

Summary

• recognize frontmatter fences accepted by the Pi SDK when they include a UTF-8 BOM, trailing horizontal whitespace, or CR-only line endings
• keep edits surgical so toggling only changes disable-model-invocation and preserves the original YAML formatting
• verify through the real skill loader that a BOM-prefixed skill remains loadable across an enable/disable round trip

The previous implementation treated these valid files as if they had no frontmatter and could prepend a second block, causing the loader to lose the skill metadata. This is a focused follow-up to #519 and preserves its original formatting-retention design.

Validation

• npm test — 846/846 passed
• node_modules/.bin/tsc --noEmit
• npm run lint
• git diff --check