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/activities.py

@@ -22,7 +22,7 @@ from temporalio.exceptions import ApplicationError
 
 from activity_core.db import make_engine
 from activity_core.orm import ActivityDefinition as ActivityDefinitionRow
-from activity_core.orm import ActivityRun
+from activity_core.orm import ActivityRun, TaskInstance
 
 
 _session_factory: async_sessionmaker[AsyncSession] | None = None
@@ -162,3 +162,41 @@ async def log_run(run_payload: dict) -> str:
             await session.execute(stmt)
 
     return str(run_id)
+
+
+@activity.defn
+async def persist_task_instance(task_payload: dict) -> str:
+    """Write a TaskInstance row and return its id.
+
+    Idempotent: uses INSERT … ON CONFLICT (id) DO NOTHING.
+
+    Expected keys in task_payload:
+        id        (str UUID — deterministic, computed in TaskExecutorWorkflow)
+        run_id    (str UUID)
+        type      (str)
+        params    (dict)
+        status    (str, default "done" for stub)
+
+    Returns:
+        task instance id as a str UUID.
+    """
+    Session = _get_session_factory()
+    task_id = uuid.UUID(task_payload["id"])
+
+    stmt = (
+        pg_insert(TaskInstance)
+        .values(
+            id=task_id,
+            run_id=uuid.UUID(task_payload["run_id"]),
+            type=task_payload["type"],
+            params=task_payload.get("params", {}),
+            status=task_payload.get("status", "done"),
+        )
+        .on_conflict_do_nothing(index_elements=["id"])
+    )
+
+    async with Session() as session:
+        async with session.begin():
+            await session.execute(stmt)
+
+    return str(task_id)