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/openai_compatible_tts.py

@@ -49,7 +49,7 @@ class OpenAICompatibleTTSService(BaseTTSService):
         Initialize OpenAI-compatible TTS service.
         
         Args:
-            api_key: Provider API key (defaults to TTS_API_KEY/SILICONFLOW_API_KEY env vars)
+            api_key: Provider API key
             api_url: Provider API URL (defaults to SiliconFlow endpoint)
             voice: Voice name (alex, anna, bella, benjamin, charles, claire, david, diana)
             model: Model name
@@ -73,7 +73,7 @@ class OpenAICompatibleTTSService(BaseTTSService):
             
         super().__init__(voice=full_voice, sample_rate=sample_rate, speed=speed)
         
-        self.api_key = api_key or os.getenv("TTS_API_KEY") or os.getenv("SILICONFLOW_API_KEY")
+        self.api_key = api_key
         self.model = model
         raw_api_url = api_url or os.getenv("TTS_API_URL") or "https://api.siliconflow.cn/v1/audio/speech"
         self.api_url = self._resolve_speech_endpoint(raw_api_url)