Function run_decision_surface 

pub fn run_decision_surface(opts: &AuditOptions<'_>) -> ExitCode

Run the standalone fallow decision-surface command: the separable, cheap apex. Executes the SAME changed-code analysis the review brief runs (it is the brief path, NOT the full project pipeline), then emits ONLY the decision surface envelope. Always exit 0 (the surface is advisory, never a gate).

The MCP decision_surface tool wraps this command. It is callable without the full pipeline because it reuses execute_audit in brief mode (changed-code scope), not bare fallow.