- Add Provider field to AgentDefaults struct
- Modify CreateProvider to use explicit provider field first, fallback to model name detection
- Allows using models without provider prefix (e.g., llama-3.1-8b-instant instead of groq/llama-3.1-8b-instant)
- Supports all providers: groq, openai, anthropic, openrouter, zhipu, gemini, vllm
- Backward compatible with existing configs
Fixes issue where models without provider prefix could not use configured API keys.
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