Completes the model re-tiering from #2442: the gan-planner, gan-generator,
and gan-evaluator agents were already re-pinned to sonnet, but the
gan-style-harness script and docs still defaulted GAN_PLANNER_MODEL,
GAN_GENERATOR_MODEL, and GAN_EVALUATOR_MODEL to opus. Align the script
defaults, skill docs (en/ja/zh), and example commands with the landed
agent tiers. Opus remains available via the existing env overrides.
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>