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.

services/memory/skills.py

@@ -472,24 +472,29 @@ class SkillsManager:
     # Reading a single skill (used by the skill_view tool)
     # ----------------------------------------------------------------------
 
-    def read_skill_md(self, name: str) -> Optional[str]:
+    def read_skill_md(self, name: str, owner: Optional[str] = None) -> Optional[str]:
         for path in self._iter_skill_files():
             sk = self._read_skill(path)
-            if sk and sk.name == name:
-                try:
-                    with open(path, encoding="utf-8") as f:
-                        return f.read()
-                except Exception:
-                    return None
+            if not sk or sk.name != name:
+                continue
+            if (sk.owner or "") != (owner or ""):
+                continue
+            try:
+                with open(path, encoding="utf-8") as f:
+                    return f.read()
+            except Exception:
+                return None
         return None
 
-    def read_skill_reference(self, name: str, ref_path: str) -> Optional[str]:
+    def read_skill_reference(self, name: str, ref_path: str, owner: Optional[str] = None) -> Optional[str]:
         """Read a sub-file under the skill's directory (references/, etc).
         Refuses path traversal."""
         for path in self._iter_skill_files():
             sk = self._read_skill(path)
             if not sk or sk.name != name:
                 continue
+            if (sk.owner or "") != (owner or ""):
+                continue
             base = os.path.realpath(os.path.dirname(path))
             target = os.path.realpath(os.path.join(base, ref_path))
             if os.path.commonpath([base, target]) != base or target == os.path.dirname(path):