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.

src/tool_implementations.py

@@ -651,7 +651,7 @@ async def do_manage_skills(content: str, owner: Optional[str] = None) -> Dict:
     if action == "view":
         if not name:
             return {"error": "name is required for view", "exit_code": 1}
-        md = sm.read_skill_md(name)
+        md = sm.read_skill_md(name, owner=owner)
         if md is None:
             return {"error": f"Skill {name!r} not found", "exit_code": 1}
         return {"results": md}
@@ -662,7 +662,7 @@ async def do_manage_skills(content: str, owner: Optional[str] = None) -> Dict:
         ref = (args.get("path") or "").strip()
         if not ref:
             return {"error": "path is required for view_ref", "exit_code": 1}
-        text = sm.read_skill_reference(name, ref)
+        text = sm.read_skill_reference(name, ref, owner=owner)
         if text is None:
             return {"error": f"Reference {ref!r} not found under {name!r}", "exit_code": 1}
         return {"results": text}
@@ -747,7 +747,7 @@ async def do_manage_skills(content: str, owner: Optional[str] = None) -> Dict:
         new_str = args.get("new_string", "")
         if not isinstance(old, str) or not old:
             return {"error": "old_string is required and must be non-empty", "exit_code": 1}
-        md = sm.read_skill_md(name)
+        md = sm.read_skill_md(name, owner=owner)
         if md is None:
             return {"error": f"Skill {name!r} not found", "exit_code": 1}
         count = md.count(old)