feat: add session updates and message dialogs

backend/services/brains/base.py

@@ -44,6 +44,14 @@ class BrainSpec:
     owns_context: bool
 
 
+@dataclass(frozen=True)
+class SessionVariableUpdate:
+    """Result of one silent update to declared session variables."""
+
+    changed: list[str]
+    dynamic_variables: dict[str, str | int | float | bool]
+
+
 class CallEndPort(Protocol):
     """Small call-lifecycle surface available to a brain."""
 
@@ -144,6 +152,13 @@ class BaseBrain:
     async def on_client_ready(self) -> None:
         """Replay client-visible state after its app message channel is ready."""
 
+    async def on_session_update(
+        self,
+        dynamic_variables: dict[str, Any],
+    ) -> SessionVariableUpdate:
+        """Apply a silent client state update without starting an LLM turn."""
+        raise ValueError(f"助手类型 {self.spec.type} 不支持会话动态变量更新")
+
     def record_user_message(self, content: str) -> None:
         """Observe a committed user message for brain-owned routing state."""
 
@@ -194,6 +209,11 @@ class Brain(Protocol):
 
     async def on_client_ready(self) -> None: ...
 
+    async def on_session_update(
+        self,
+        dynamic_variables: dict[str, Any],
+    ) -> SessionVariableUpdate: ...
+
     def record_user_message(self, content: str) -> None: ...
 
     async def on_user_turn_end(self, content: str) -> bool: ...