add the config and compose modules

config: dotenv files are parsed, never sourced, so a value can hold a # or a
$(...) without breaking or executing. The make engine included .env with
make syntax, which neither allowed.
compose: one call per project instead of one per sub-stack.

spec/unit/config_load_helper.sh

@@ -0,0 +1,5 @@
+#shellcheck shell=sh
+# Helper: myos_dotenv_load sets variables in the caller's scope.
+[ -n "${2:-}" ] && LOADED=$2
+myos_dotenv_load "$1"
+printf '%s\n' "$LOADED"