move vad config to json

config.json

@@ -12,6 +12,15 @@
   "session": {
     "inactivity_timeout_sec": 60
   },
+  "turn": {
+    "vad": {
+      "confidence": 0.7,
+      "start_secs": 0.2,
+      "stop_secs": 0.4,
+      "min_volume": 0.6
+    },
+    "user_speech_timeout_sec": 0.8
+  },
   "agent": {
     "system_prompt": "You are a helpful, friendly voice assistant. Keep responses concise and natural for spoken conversation.",
     "greeting": "Please introduce yourself briefly.",