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.

requirements.txt

@@ -3,6 +3,7 @@ uvicorn[standard]>=0.34.0
 gunicorn>=22.0.0
 langchain>=0.3.0
 langchain-openai>=0.3.0
+litellm>=1.50.0
 pydantic>=2.10.0
 pydantic-settings>=2.7.0
 python-jose[cryptography]>=3.3.0