Function config_from_env 

pub fn config_from_env() -> KhiveConfig

Build an in-memory KhiveConfig from the legacy env-var path.

Used when no config file is present. Emits tracing::info! directing operators to migrate to ~/.khive/config.toml.

The primary model (KHIVE_EMBEDDING_MODEL) becomes the default = true engine; additional models become non-default secondary engines.