Add src/brief_state.py

src/brief_state.py

@@ -0,0 +1,8 @@
+from typing import TypedDict, List, Optional
+
+class BriefState(TypedDict):
+    topic: str
+    outline: List[str] | None
+    step_index: int
+    notes: List[str]
+    final_brief: str | None