fix(skills): scope skill reads to caller owner (#777)

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.

routes/skills_routes.py

@@ -766,7 +766,7 @@ async def _audit_one_skill(skills_manager, skill, url, model, headers,
         except Exception:
             pass
 
-    md = skills_manager.read_skill_md(name)
+    md = skills_manager.read_skill_md(name, owner=owner)
     if not md:
         log(f"{name}: no source — skipped")
         return {"skill": name, "result": "skipped"}
@@ -1246,7 +1246,7 @@ def setup_skills_routes(skills_manager: SkillsManager) -> APIRouter:
         if not match:
             raise HTTPException(404, "Skill not found")
         _verify_owner(match, user)
-        md = skills_manager.read_skill_md(match.get("name"))
+        md = skills_manager.read_skill_md(match.get("name"), owner=user)
         if md is None:
             raise HTTPException(404, "Skill source unavailable (legacy entry?)")
         return {"name": match.get("name"), "markdown": md}
@@ -1273,7 +1273,7 @@ def setup_skills_routes(skills_manager: SkillsManager) -> APIRouter:
             raise HTTPException(404, "Skill not found")
         _verify_owner(match, user)
         name = match.get("name")
-        md = skills_manager.read_skill_md(name) or ""
+        md = skills_manager.read_skill_md(name, owner=user) or ""
 
         if not task:
             task = _skill_test_task(match)