feat: add get_current_state workflow query to BaseWorkflow

Adds a default @workflow.query handler named "get_current_state"
to BaseWorkflow. Returns "unknown" by default — agents override
to report their actual state.

Example for StateMachine-based agents:

    @workflow.query(name="get_current_state")
    def get_current_state(self) -> str:
        return self.state_machine.get_current_state()

Companion to scaleapi/scale-agentex#173 which adds
GET /tasks/{task_id}/query/{query_name} endpoint.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: gabrycina

src/agentex/lib/core/temporal/workflows/workflow.py

@@ -16,6 +16,22 @@ def __init__(
     ):
         self.display_name = display_name
 
+    @workflow.query(name="get_current_state")
+    def get_current_state(self) -> str:
+        """Query handler for the current workflow state.
+
+        Returns "unknown" by default. Subclasses should override this
+        to return their actual state, enabling external callers to
+        detect turn completion.
+
+        Example override for StateMachine-based agents:
+
+            @workflow.query(name="get_current_state")
+            def get_current_state(self) -> str:
+                return self.state_machine.get_current_state()
+        """
+        return "unknown"
+
     @abstractmethod
     @workflow.signal(name=SignalName.RECEIVE_EVENT)
     async def on_task_event_send(self, params: SendEventParams) -> None: