add shellspec golden harness for the legacy make engine

- spec/golden/cases.txt: 69 black-box cases (CLI wrapper mode and make
  include mode) run in a hermetic sandbox with a mocked docker
- spec/golden/expected/*.txt recorded from this engine (baseline)
- make test / test-golden / golden-record / lint dev targets

spec/spec_helper.sh

@@ -0,0 +1,6 @@
+#shellcheck shell=sh
+# shellspec helper: loaded before every spec file.
+MYOS_ROOT=${MYOS_ROOT:-$SHELLSPEC_PROJECT_ROOT}
+SPEC_DIR=$MYOS_ROOT/spec
+export MYOS_ROOT SPEC_DIR
+. "$SPEC_DIR/support/run.sh"