read_skill_md and read_skill_reference walk all skill files via
_iter_skill_files and return the first match by slug, regardless
of owner. In a multi-user deployment where two users have skills
with the same slug under different categories, a caller scoped
to owner='alice' can read Bob's skill content.
This is the same cross-tenant leak class as the update_skill /
delete_skill fix (PR #755, merged), but on the read path.
Changes:
- read_skill_md / read_skill_reference accept owner= param (default
None = match ownerless only, matching the write-path convention).
- 7 callers updated: tool_implementations.py (view, view_ref, patch),
builtin_actions.py (test_skills), skills_routes.py (audit, source,
test routes).
- Tests: read scoping (alice reads hers, not bob's), positive update
scoping (alice can mutate her own), ownerless-match default.
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