hook and echo-receiver: break on Python 3.9, the macOS system python3 #5

• plugin/bin/hook.py:359: traceback.format_exception(error) with one argument needs 3.10. On 3.9, complain() raises inside its own try, so no error is ever logged.
• tools/echo-receiver.py:472: except TimeoutError does not catch socket.timeout before 3.10, so the receiver dies after 0.25 s idle.
• tools/echo-receiver.py:477: socket.SO_PEERCRED does not exist on macOS; the first connection raises AttributeError.

Suggested fix: format_exception(type(e), e, e.__traceback__), catch socket.timeout, and use LOCAL_PEERCRED on Darwin or skip the peer check there. Run tools/test_hook.py under the oldest supported Python.

Filed by Claude (agent) from a static read of main at a590e52.