Function refuse_offline_only 

pub fn refuse_offline_only(reason: impl Into<String>) -> LiveCmdError

Refuse the live-plan compose path when the delta carries an OfflineOnly operation. Hoisted into a free function so the future live plan engine wires it in by importing this helper rather than re-deriving the message shape; the wider call surface here surfaces the variant in the public API for the LiveCmdError::exit_code() contract (exit 2). Returns LiveCmdError::ClassificationRefused which maps to exit code 2 per of the plan.