Soporte base_url custom en OpenAI adapter (MiniMax, DeepInfra, etc.)

Nueva variable AGENTIC_OPENAI_BASE_URL para proveedores compatibles
con OpenAI API (MiniMax, DeepInfra, Together, etc.).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/config.py

@@ -30,6 +30,7 @@ class Settings(BaseSettings):
     # --- Model providers ---
     anthropic_api_key: str = ""
     openai_api_key: str = ""
+    openai_base_url: str = ""  # Custom base URL (for MiniMax, DeepInfra, etc.)
     default_model_provider: str = "claude"
     default_model_id: str = "claude-sonnet-4-20250514"
     max_tokens: int = 4096