Fix `NoMethodError: undefined method 'articles'` on game_sessions#show

Fix NoMethodError: undefined method 'articles' for an instance of Campaign on game_sessions#show. BL-045’s lore tab called @campaign.articles but Campaign has no has_many :articles association — articles belongs_to :campaign, optional: true only. Fix mirrors the existing lore action pattern: Article.where(campaign: @campaign). Honeybadger #130037210.