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/llm.py

@@ -44,13 +44,13 @@ class OpenAILLMService(BaseLLMService):
         
         Args:
             model: Model name (e.g., "gpt-4o-mini", "gpt-4o")
-            api_key: Provider API key (defaults to LLM_API_KEY/OPENAI_API_KEY env vars)
+            api_key: Provider API key
             base_url: Custom API base URL (for Azure or compatible APIs)
             system_prompt: Default system prompt for conversations
         """
         super().__init__(model=model)
         
-        self.api_key = api_key or os.getenv("LLM_API_KEY") or os.getenv("OPENAI_API_KEY")
+        self.api_key = api_key
         self.base_url = base_url or os.getenv("LLM_API_URL") or os.getenv("OPENAI_API_URL")
         self.system_prompt = system_prompt or (
             "You are a helpful, friendly voice assistant. "