A decade or so ago, the premise of “ O Horizon ” might have seemed like “Black Mirror” fodder: Fed various photos, videos, messages and personal effects of a dead man, a computer program devises an interactive simulacrum, available at any time for conversation via a phone app, live if not alive. Today, the idea doesn’t feel old hat so much as depressingly immediate, as discussions of the ethical and existential ramifications of AI chatbots have migrated from the hypothetical to the everyday.