Skills: delete owner-scoped skills with owner

The DELETE /api/skills/{skill_id} handler resolves the caller, loads the
skill with skills_manager.load(owner=user), and verifies ownership with
_verify_owner(match, user) — but then calls
skills_manager.delete_skill(match.get("name")) without the owner.

SkillsManager.delete_skill filters candidates with
`(sk.owner or "") != (owner or "")`, so when owner is None an owner-scoped
skill is skipped and the method returns False. The route then raises a
spurious 404 "Skill not found" — meaning a logged-in user can never delete
their own skills through the API.

Pass the resolved owner through to delete_skill so the skill is matched and
removed.

tests/test_skills_delete_owner.py drops a real owner-scoped SKILL.md on disk
and (1) checks the manager directly: delete_skill without owner returns
False (regression lock) while delete_skill(owner="alice") returns True and
removes the dir; (2) drives the real DELETE route handler and asserts it
returns {"ok": True} and deletes the file. The route test fails before this
change (404). Real SkillsManager + real filesystem, no mocking.

routes/skills_routes.py

@@ -1511,7 +1511,7 @@ def setup_skills_routes(skills_manager: SkillsManager) -> APIRouter:
         if not match:
             raise HTTPException(404, "Skill not found")
         _verify_owner(match, user)
-        ok = skills_manager.delete_skill(match.get("name"))
+        ok = skills_manager.delete_skill(match.get("name"), owner=user)
         if not ok:
             raise HTTPException(404, "Skill not found")
         return {"ok": True}