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

.shellspec

@@ -0,0 +1,4 @@
+--require spec_helper
+--shell bash
+--default-path spec/unit:spec/golden
+--pattern "*_spec.sh"