Background-task status narration prompt now sees the previous line

The prompt that generates the one-line status text shown while a background or agent task runs now receives the previously-shown line and is told to repeat it verbatim if it's still accurate, only changing it when something actually happened or the goal moved. Its system prompt also adds explicit style rules: a 14-word maximum, no file paths or commands unless the user typed them, and describing the goal rather than mechanical steps.