Closes the functional gap the reporter hit on alchaincyf/huashu-design
(skills.sh/alchaincyf/huashu-design/huashu-design) without expanding
candidatePaths unconditionally, which would let an unrelated root
SKILL.md hijack a different skill URL in a multi-skill repo.
Try SKILL.md at the repo root before falling into the recursive tree
fallback added in #1432. Verify the frontmatter name matches the
requested skill so only genuine single-skill repos take the fast path.
For those repos this also shaves the recursive tree API call.
Also clarifies the candidate-path comment so the root case is
explicit.
