fix(spawner): A13 exit=0 always completed, not agent_error

exit=0 means process exited normally. Trust the exit code regardless
of stdout/JSON output or task_status. Old logic misclassified inform
Mail completions as agent_error, causing infinite retry loops.

Includes test update: test_task_status_pending expects completed.

tests/unit/test_classify_outcome.py

@@ -123,7 +123,7 @@ class TestClassifyNoJsonExit0:
 
     def test_task_status_pending(self):
         result = Spawner._classify_outcome(0, {}, "", "pending", "")
-        assert result["outcome"] == "agent_error"
+        assert result["outcome"] == "completed"
         assert result["should_retry"] is False