feat(workflows): TaskExecutorWorkflow stub + wire worker — T19/T20

activities.py — persist_task_instance (new):
  Idempotent INSERT ... ON CONFLICT (id) DO NOTHING on task_instances.
  task_id passed in from workflow (derived from workflow_id via uuid5).
  Registered on task-execution-tq.

workflows.py — TaskExecutorWorkflow (T19):
  Derives stable task_id = uuid5(NAMESPACE_URL, workflow_id).
  Calls persist_task_instance → status=done, returns immediately.
  Real execution logic to replace stub in a later workstream.

worker.py — T20:
  Registers persist_task_instance on task-execution-tq Worker.
  Both queues fully wired: orchestrator-tq and task-execution-tq.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/activity_core/workflows.py

@@ -21,6 +21,7 @@ with workflow.unsafe.imports_passed_through():
     from activity_core.activities import (
         load_activity_definition,
         log_run,
+        persist_task_instance,
         resolve_context,
     )
     from activity_core.template_engine import evaluate_templates
@@ -123,9 +124,38 @@ class RunActivityWorkflow:
 class TaskExecutorWorkflow:
     """Child workflow that executes one concrete task instance.
 
-    Stub implementation — T19.
+    Stub behaviour: persists a task_instances row with status=done and
+    returns immediately. Real task execution logic replaces this in a
+    later workstream.
+
+    task_id is derived deterministically from the workflow's own ID so
+    persist_task_instance retries remain idempotent.
     """
 
     @workflow.run
     async def run(self, run_id: str, task_type: str, params: dict) -> dict:
-        raise NotImplementedError("T19")
+        # Derive a stable task_id from this workflow's own ID.
+        task_id = str(
+            uuid.uuid5(uuid.NAMESPACE_URL, workflow.info().workflow_id)
+        )
+
+        workflow.logger.info(
+            "TaskExecutorWorkflow started",
+            extra={"run_id": run_id, "task_type": task_type, "task_id": task_id},
+        )
+
+        await workflow.execute_activity(
+            persist_task_instance,
+            {
+                "id": task_id,
+                "run_id": run_id,
+                "type": task_type,
+                "params": params,
+                "status": "done",
+            },
+            task_queue=_TASK_QUEUE,
+            start_to_close_timeout=_ACTIVITY_TIMEOUT,
+            retry_policy=_RETRY_POLICY,
+        )
+
+        return {"task_id": task_id, "status": "done"}