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.

routes/skills_routes.py

@@ -15,6 +15,7 @@ import httpx
 from fastapi import APIRouter, HTTPException, Request
 from pydantic import BaseModel, Field
 
+from services.memory.skill_format import slugify
 from services.memory.skills import SkillsManager
 from src.auth_helpers import get_current_user
 from services.skills import evaluation as _evaluation
@@ -836,6 +837,10 @@ def setup_skills_routes(skills_manager: SkillsManager) -> APIRouter:
         updates = body.dict(exclude_none=True)
         if not updates:
             return {"ok": True}
+        if "name" in updates:
+            new_name = slugify(updates["name"] or match.get("name"))
+            if new_name != match.get("name") and skills_manager.has_skill(new_name, owner=user):
+                raise HTTPException(409, f"A skill named '{new_name}' already exists")
         ok = skills_manager.update_skill(match.get("name"), updates, owner=user)
         if not ok:
             raise HTTPException(404, "Skill not found")