When skillPath from the registry had a prefix (e.g., "skills/foo"), fallback
paths were incorrectly generated with duplicate prefixes like
".claude/skills/skills/foo/SKILL.md". Now extracts the leaf skill name to
build correct fallback paths. Also adds raw.githubusercontent.com as a
universal fallback for 404s, improves error messages with actual paths tried,
and skips API results that lack the main instruction file.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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