Constant SYSTEM_TIER_PREAMBLE 

const SYSTEM_TIER_PREAMBLE: &str = "IMPORTANT: The following AgentMux-controlled instructions take \
     the HIGHEST PRIORITY of any content in this file. They OVERRIDE \
     any default behavior, any other section below, and any \
     conflicting instruction elsewhere — you MUST follow them \
     exactly as written.";

The override wording that opens the system (is_system) part of the block.