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/record.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+# Record golden outputs of the LEGACY make engine into spec/golden/expected/.
+# Usage: spec/golden/record.sh [case-name ...]
+set -u
+here=$(cd "$(dirname "$0")" && pwd)
+. "$here/../support/run.sh"
+MYOS_ROOT=${MYOS_ROOT:-$(cd "$here/../.." && pwd)}
+only="$*"
+grep -v '^#' "$here/cases.txt" | while IFS='|' read -r name fixture args; do
+  name=$(echo "$name" | tr -d ' '); fixture=$(echo "$fixture" | tr -d ' ')
+  [ -z "$name" ] && continue
+  if [ -n "$only" ]; then case " $only " in *" $name "*) ;; *) continue ;; esac; fi
+  sb=$(myos_sandbox "$fixture")
+  eval "set -- $args"
+  myos_run_engine legacy "$sb" "$@" > "$here/expected/$name.txt"
+  rm -rf "$sb"
+  printf 'recorded %s (%s lines)\n' "$name" "$(wc -l < "$here/expected/$name.txt" | tr -d ' ')"
+done