Start LangGraph backend migration foundation

src/agent/state.py

@@ -0,0 +1,30 @@
+"""Minimal serializable state for the first LangGraph slice."""
+
+from dataclasses import dataclass, field
+from typing import Any, Protocol, TypedDict
+
+
+class AccidentGraphState(TypedDict, total=False):
+    session_id: str
+    input_text: str
+    need_form_update: bool
+    response_text: str
+    stage_code: str
+    form_update: dict[str, Any]
+    turn_count: int
+
+
+@dataclass(frozen=True)
+class GeneratedTurn:
+    content: str
+    form_update: dict[str, Any] = field(default_factory=dict)
+
+
+class ResponseGenerator(Protocol):
+    async def generate(self, state: AccidentGraphState) -> GeneratedTurn:
+        """Generate one state-prefixed response from graph state."""
+        ...
+
+    async def aclose(self) -> None:
+        """Release any owned network resources."""
+        ...