fix(skills): refuse renaming a skill onto a name the owner already uses (#6941)

update_skill only checked whether the target directory existed. A skill
with that name in another category lives elsewhere on disk, so the
rename went through and left two skills with one name. Update and
delete look skills up by name, so the next delete removed whichever
file os.walk reached first.

Check the owner's skills in every category before renaming, and answer
409 from PUT /api/skills/{id} instead of the misleading 404.

services/memory/skills.py

@@ -500,6 +500,19 @@ class SkillsManager:
         sk.path = self._skill_file(cat, nm)
         return sk.to_dict()
 
+    def has_skill(self, name: str, owner: Optional[str] = None) -> bool:
+        """Whether `owner` already has a skill called `name`, in any category.
+
+        Update and delete find a skill by name within its owner, so two of
+        one owner's skills sharing a name would make them act on whichever
+        file is walked first.
+        """
+        for path in self._iter_skill_files():
+            sk = self._read_skill(path)
+            if sk and sk.name == name and (sk.owner or "") == (owner or ""):
+                return True
+        return False
+
     def update_skill(self, skill_id: str, updates: Dict, owner: Optional[str] = None) -> bool:
         """`skill_id` is the slug name. Allows updating any field plus
         renames if `name` changes (file is moved on disk).
@@ -551,6 +564,9 @@ class SkillsManager:
             # Rename
             new_name = slugify(updates.get("name") or sk.name)
             if new_name != sk.name:
+                if self.has_skill(new_name, owner=owner):
+                    logger.warning(f"Skill rename target name in use: {new_name}")
+                    return False
                 sk.name = new_name
 
             # Write to potentially new path