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/app/main.py

@@ -371,12 +371,10 @@ async def startup_event():
     logger.info(f"Server: {settings.host}:{settings.port}")
     logger.info(f"Sample rate: {settings.sample_rate} Hz")
     logger.info(f"VAD model: {settings.vad_model_path}")
-    if settings.agent_config_path:
-        logger.info(
-            f"Agent config loaded ({settings.agent_config_source}): {settings.agent_config_path}"
-        )
-    else:
-        logger.info("Agent config: none (using .env/default agent values)")
+    logger.info(
+        "Assistant runtime config source: backend when BACKEND_URL is set, "
+        "otherwise local YAML by assistant_id from ASSISTANT_LOCAL_CONFIG_DIR"
+    )
 
 
 @app.on_event("shutdown")