* fix(skills): use Git Trees API to prevent silent subdirectory loss during install
Refactors _download_directory() to use the Git Trees API (single call
for the entire repo tree) as the primary path, falling back to the
recursive Contents API when the tree endpoint is unavailable or
truncated. Prevents silent subdirectory loss caused by per-directory
rate limiting or transient failures.
Cherry-picked from PR #2981 by tugrulguner.
Fixes#2940.
* fix: simplify tree API — use branch name directly as tree-ish
Eliminates an extra git/ref/heads API call by passing the branch name
directly to git/trees/{branch}?recursive=1, matching the pattern
already used by _find_skill_in_repo_tree.
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Co-authored-by: tugrulguner <tugrulguner@users.noreply.github.com>
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