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/services/realtime.py

@@ -13,7 +13,6 @@ The Realtime API provides:
 - Barge-in/interruption handling
 """
 
-import os
 import asyncio
 import json
 import base64
@@ -98,7 +97,6 @@ class RealtimeService:
             config: Realtime configuration (uses defaults if not provided)
         """
         self.config = config or RealtimeConfig()
-        self.config.api_key = self.config.api_key or os.getenv("OPENAI_API_KEY")
         
         self.state = RealtimeState.DISCONNECTED
         self._ws = None