Implement LangGraph integration: refactor agent-tool interaction, add graph compilation, and enhance state management

core/state.py

@@ -0,0 +1,20 @@
+"""
+LangGraph agent state — defines the shape of the state object that flows
+through every node in the agent graph.
+"""
+
+from typing import Annotated, TypedDict
+
+from langgraph.graph.message import add_messages
+
+
+class AgentState(TypedDict):
+    """
+    The single source of truth that travels through every node in the graph.
+
+    `messages` uses LangGraph's `add_messages` reducer, which:
+    - Appends new messages to the list.
+    - Replaces messages with the same ID (useful for tool-call results).
+    """
+
+    messages: Annotated[list, add_messages]