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/core/llm.py

@@ -0,0 +1,68 @@
+"""LLM factory — centralised model instantiation via LiteLLM.
+
+Every agent and the orchestrator call ``get_llm()`` or ``get_router_llm()``
+instead of directly constructing a provider-specific class.  The model string
+follows the `LiteLLM model naming convention
+<https://docs.litellm.ai/docs/providers>`_:
+
+* OpenAI:     ``gpt-4o``, ``gpt-4o-mini``
+* Anthropic:  ``anthropic/claude-3.5-sonnet``
+* Google:     ``gemini/gemini-pro``
+* Ollama:     ``ollama/llama3``
+* Bedrock:    ``bedrock/anthropic.claude-v2``
+
+Switch providers by changing **LLM_MODEL** / **LLM_ROUTER_MODEL** in ``.env``
+— no code changes required.
+"""
+
+from __future__ import annotations
+
+from langchain_openai import ChatOpenAI
+from litellm import get_supported_openai_params  # noqa: F401 – validates install
+
+from app.config.settings import settings
+
+
+def _api_key_for_model(model: str) -> str | None:
+    """Return the most appropriate API key for the given LiteLLM model string."""
+    if model.startswith("anthropic/"):
+        return getattr(settings, "ANTHROPIC_API_KEY", None) or None
+    if model.startswith("gemini/") or model.startswith("google/"):
+        return getattr(settings, "GOOGLE_API_KEY", None) or None
+    # Default: OpenAI-compatible (covers plain model names like "gpt-4o")
+    return settings.OPENAI_API_KEY or None
+
+
+def get_llm(
+    *,
+    model: str | None = None,
+    temperature: float = 0,
+) -> ChatOpenAI:
+    """Return a LangChain chat model backed by LiteLLM.
+
+    LiteLLM exposes an OpenAI-compatible API, so we use ``ChatOpenAI`` pointed
+    at the LiteLLM proxy endpoint.  In practice, ``litellm`` patches the
+    ``openai`` client transparently when the model string contains a provider
+    prefix (``anthropic/…``, ``gemini/…``, etc.).
+
+    Parameters
+    ----------
+    model:
+        LiteLLM model identifier. Defaults to ``settings.LLM_MODEL``.
+    temperature:
+        Sampling temperature.  ``0`` = deterministic.
+    """
+    model = model or settings.LLM_MODEL
+    return ChatOpenAI(
+        model=model,
+        temperature=temperature,
+        api_key=_api_key_for_model(model),
+    )
+
+
+def get_router_llm(
+    *,
+    temperature: float = 0,
+) -> ChatOpenAI:
+    """Return the lighter model used for intent classification / routing."""
+    return get_llm(model=settings.LLM_ROUTER_MODEL, temperature=temperature)