Implement final changes from the code quality and deduplication opportunity audit

internal/stage/merge.go

@@ -156,7 +156,7 @@ func (mergeStage) Run(ctx context.Context, env *Env, m *manifest.Manifest) (*Sta
 		SessionID: sessionID,
 	})
 	if err != nil {
-		return nil, fmt.Errorf("merge: promote merged transcript: %w", err)
+		return nil, fmt.Errorf("merge: materialize canonical base transcript: %w", err)
 	}
 	outputs := []artifacts.Ref{materializedMerged}
 	if reportEnabled {
@@ -166,7 +166,7 @@ func (mergeStage) Run(ctx context.Context, env *Env, m *manifest.Manifest) (*Sta
 			SessionID: sessionID,
 		})
 		if err != nil {
-			return nil, fmt.Errorf("merge: promote report: %w", err)
+			return nil, fmt.Errorf("merge: materialize canonical report: %w", err)
 		}
 		outputs = append(outputs, materializedReport)
 	}