Refactor LLM instantiation across agents and orchestrator

- Replaced direct instantiation of ChatOpenAI with a centralized get_llm function in CheckpointAgent, NoteAgent, ProjectAgent, and TaskAgent.
- Introduced a new llm.py module to handle LLM model instantiation and API key management.
- Updated settings.py to include LLM_MODEL and LLM_ROUTER_MODEL configurations.
- Modified orchestrator.py to use get_router_llm for intent classification.
- Updated requirements.txt to include litellm for LLM management.
- Adjusted tests to mock get_llm instead of ChatOpenAI directly.

app/config/settings.py

@@ -24,6 +24,9 @@ class Settings(BaseSettings):
 
     OPENAI_API_KEY: str = ""
 
+    LLM_MODEL: str = "gpt-4o"
+    LLM_ROUTER_MODEL: str = "gpt-4o-mini"
+
     CORS_ORIGINS: list[str] = ["app://.", "http://localhost:3000", "http://localhost:5173"]
 
     ENV: Literal["dev", "prod"] = "dev"