fix(s11): document unreachable else branch in inject_background_results (#584)

The else branch in inject_background_results is never reached in the
current agent_loop call pattern, where messages[-1] is always a user
message. Kept for independent reuse or future wake-up scenarios.

s11_background_tasks/code.py

@@ -460,6 +460,9 @@ def inject_background_results(messages: list) -> int:
                 *blocks,
             ]
     else:
+        # Currently unreachable: agent_loop always leaves messages[-1] as a
+        # user message, so this branch is never hit in the teaching code.
+        # Kept for independent reuse or future wake-up from background tasks.
         messages.append({"role": "user", "content": blocks})
     return len(notifications)