🔧(agents) make Silero VAD optional

Allow configuring whether a VAD model runs before calling an external ASR API.
Running VAD can save API calls (and costs) when no audible sound is detected,
but comes with the trade-off of additional computational overhead.

src/helm/env.d/dev-keycloak/values.meet.yaml.gotmpl

@@ -267,6 +267,7 @@ agents:
     LIVEKIT_API_KEY: {{ $key }}
     {{- end }}
     {{- end }}
+    ENABLE_SILERO_VAD: "false"
 
   image:
     repository: localhost:5001/meet-agents