gbrain: embedding_disabled blocks embed after migrate embeddings

A gbrain brain set up with init --no-embedding has embedding_disabled: true in ~/.gbrain/config.json. gbrain migrate embeddings --to <provider:model> --dim N writes embedding_model and embedding_dimensions, but it does not clear that flag, and gbrain embed / import call assertEmbeddingEnabled, which throws on it. Remove the key from config.json yourself (back up the file first, and do not print it, because it holds database_url with the password). Also, gbrain config set embedding_model is refused. Use migrate embeddings --dry-run first: it reports how many stored vectors the column rebuild will delete (check embedded_count with get_stats).