feat(memory): add rewound kwarg to on_session_switch hook

agent/memory_provider.py

@@ -178,6 +178,7 @@ class MemoryProvider(ABC):
         *,
         parent_session_id: str = "",
         reset: bool = False,
+        rewound: bool = False,
         **kwargs,
     ) -> None:
         """Called when the agent switches session_id mid-process.
@@ -207,6 +208,10 @@ class MemoryProvider(ABC):
             (``_session_turns``, ``_turn_counter``, etc.) when this is
             set. ``False`` for ``/resume`` / ``/branch`` / compression
             where the logical conversation continues under the new id.
+        rewound:
+            ``True`` if session_id is unchanged but the transcript was
+            truncated; providers caching per-turn document state should
+            invalidate.
 
         Default is no-op for backward compatibility.
         """