Constant TOOL_CALL_KEY 

pub const TOOL_CALL_KEY: &'static str;

Written when the governed flow admits a tool call, before it runs: {"tool": name, "id": call id}. With TOOL_DENIED_KEY and TOOL_RESULT_KEY it puts every governance decision about a tool in the mutation journal, in order, which is what makes a recorded session replayable as a scenario.