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/.env.example

@@ -30,21 +30,9 @@ CHUNK_SIZE_MS=20
 DEFAULT_CODEC=pcm
 MAX_AUDIO_BUFFER_SECONDS=30
 
-# Agent profile selection (optional fallback when CLI args are not used)
-# Prefer CLI:
-#   python -m app.main --agent-config config/agents/default.yaml
-#   python -m app.main --agent-profile default
-# AGENT_CONFIG_PATH=config/agents/default.yaml
-# AGENT_PROFILE=default
-AGENT_CONFIG_DIR=config/agents
-
-# Optional: provider credentials referenced from YAML, e.g. ${LLM_API_KEY}
-# LLM_API_KEY=your_llm_api_key_here
-# LLM_API_URL=https://api.openai.com/v1
-# TTS_API_KEY=your_tts_api_key_here
-# TTS_API_URL=https://api.example.com/v1/audio/speech
-# ASR_API_KEY=your_asr_api_key_here
-# ASR_API_URL=https://api.example.com/v1/audio/transcriptions
+# Local assistant/agent YAML directory. In local mode the runtime resolves:
+#   ASSISTANT_LOCAL_CONFIG_DIR/<assistant_id>.yaml
+ASSISTANT_LOCAL_CONFIG_DIR=engine/config/agents
 
 # Logging
 LOG_LEVEL=INFO