mail ack: usage text documents a positional agent the parser ignores #901

tps mail ack's usage text says the agent can be given as a positional [agent], but the parser accepts only --agent <id>; a positional agent is ignored and the default agent is used. Found in the review of #900.

What the fix does: the usage text and the parser agree: either the usage shows --agent <id>, or the parser accepts the documented positional form and treats it identically. A test runs the documented form through the real CLI and asserts it acks the named agent's message.