Simplify downstream tool configuration

internal/config/scriptorium_test.go

@@ -49,11 +49,19 @@ func TestScriptoriumLoadAndValidate(t *testing.T) {
 			wantLoadErr: "strict decode failed",
 		},
 		{
-			name: "missing binary fails when section present",
+			name: "missing binary defaults when section present",
 			scriptoriumYAML: `scriptorium:
   timeout: 10m
 `,
-			wantValidateErr: "pipeline.scriptorium.binary is required",
+			assert: func(t *testing.T, cfg *Config) {
+				t.Helper()
+				if cfg.Pipeline.Scriptorium == nil {
+					t.Fatal("scriptorium config should be present")
+				}
+				if cfg.Pipeline.Scriptorium.Binary != "scriptorium" {
+					t.Fatalf("scriptorium.binary = %q, want scriptorium", cfg.Pipeline.Scriptorium.Binary)
+				}
+			},
 		},
 		{
 			name: "enabled artifact missing prompt id fails",