Replace the 'single'/'carousel' string format threaded through the AI
generation path with a typed GeneratorFormat enum (PostContentGenerator,
PostContentHumanizer, PostContentStreamer, StreamPostCreation,
RunGenerateNode, deriveFormat).
Remove generatorFormat() from AiContentTemplate and the three templates —
it conflated style and format (returning 'tweet_card'). The tweet-card
check now lives on ContentStyle via isTweetCard(), and the humanizer skip
on ContentStyle::humanizes(), so the style declares its own behaviour
instead of callers comparing magic strings.
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