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/golden/DELTAS.md

@@ -0,0 +1,8 @@
+# Intentional differences between the legacy make engine and the bash CLI
+
+Golden expectations in `expected/` are recorded from the legacy engine
+(`git tag legacy-1.0-beta`). When the CLI intentionally behaves differently,
+the case gets an override in `expected.cli/<case>.txt` and a line here.
+
+| case | delta | why |
+|---|---|---|