review: tell users to update a pi too old for isolated reviews

A pi that predates --no-approve (0.70.2, and every release of the old @mariozechner/pi-coding-agent package, up to 0.73.1) does not merely warn about it: it prints "Unknown option: --no-approve" and exits 1, so every pi review on it failed with a bare option error. Failing closed is right — dropping the flag would load the branch's .pi/settings.json — but the review now says which flag is missing and to update pi, via the reviewer's ClassifyExit hook. Corrects the comment that said such releases only warn.