feat: add sovereign conversation artifacts slice (#1117)

tests/test_conversation_artifacts.py

@@ -1,3 +1,7 @@
+from pathlib import Path
+
+import yaml
+
 from nexus.mempalace.config import CORE_ROOMS
 from nexus.mempalace.conversation_artifacts import (
     ConversationArtifact,
@@ -9,6 +13,8 @@ from nexus.mempalace.conversation_artifacts import (
 
 def test_sovereign_room_is_core_room() -> None:
     assert "sovereign" in CORE_ROOMS
+    rooms_yaml = yaml.safe_load(Path("mempalace/rooms.yaml").read_text())
+    assert any(room["key"] == "sovereign" for room in rooms_yaml["core_rooms"])
 
 
 def test_normalize_speaker_maps_alexander_variants() -> None: