Add CLI and service skeleton

internal/decision/engine.go

@@ -154,6 +154,12 @@ func (e *Engine) Explain(decisionID string) (Explanation, error) {
 		return Explanation{}, fmt.Errorf("decision %q not found", decisionID)
 	}
 
+	return ExplainEnvelope(decision), nil
+}
+
+// ExplainEnvelope returns the same explanation shape for an already-loaded
+// decision envelope.
+func ExplainEnvelope(decision api.DecisionEnvelope) Explanation {
 	return Explanation{
 		DecisionID:    decision.ID,
 		Effect:        decision.Effect,
@@ -166,7 +172,7 @@ func (e *Engine) Explain(decisionID string) (Explanation, error) {
 		MatchedRule:   decision.MatchedRule,
 		Diagnostics:   decision.Diagnostics,
 		Caring:        decision.Caring,
-	}, nil
+	}
 }
 
 type registryFacts struct {