Refactor assistant configuration management and update documentation

- Removed legacy agent profile settings from the .env.example and README, streamlining the configuration process.
- Introduced a new local YAML configuration adapter for assistant settings, allowing for easier management of assistant profiles.
- Updated backend integration documentation to clarify the behavior of assistant config sourcing based on backend URL settings.
- Adjusted various service implementations to directly utilize API keys from the new configuration structure.
- Enhanced test coverage for the new local YAML adapter and its integration with backend services.

engine/examples/wav_client.py

@@ -58,7 +58,7 @@ class WavFileClient:
         url: str,
         input_file: str,
         output_file: str,
-        assistant_id: str = "assistant_demo",
+        assistant_id: str = "default",
         channel: str = "wav_client",
         sample_rate: int = 16000,
         chunk_duration_ms: int = 20,
@@ -520,7 +520,7 @@ async def main():
     )
     parser.add_argument(
         "--assistant-id",
-        default="assistant_demo",
+        default="default",
         help="Assistant identifier used in websocket query parameter"
     )
     parser.add_argument(