refactor(scripts): organize script entry points

scripts/tests/test-install-guards.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+set -euo pipefail
+summary=false
+while [ $# -gt 0 ]; do
+  case "$1" in
+    --summary) summary=true; shift ;;
+    *) printf 'Unknown argument: %s\n' "$1" >&2; exit 1 ;;
+  esac
+done
+
+root=$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")/../.." && pwd)
+. "$root/scripts/lib/manifest.sh"
+. "$root/scripts/lib/install-targets.sh"
+
+work=$(mktemp -d)
+trap 'rm -rf -- "$work"' EXIT
+
+installed="$work/installed"
+missing="$work/missing"
+mkdir -p "$installed"
+: > "$(manifest_path "$installed")"
+
+any_manifest_present "$installed" "$missing" || { printf 'Expected an installed destination to be detected.\n' >&2; exit 1; }
+! any_manifest_present "$missing" || { printf 'A destination without a manifest must not count as installed.\n' >&2; exit 1; }
+! all_manifests_present "$installed" "$missing" || { printf 'Update guard must fail when any selected destination is not installed.\n' >&2; exit 1; }
+all_manifests_present "$installed" || { printf 'Update guard must pass when every selected destination is installed.\n' >&2; exit 1; }
+
+if [ "$summary" = true ]; then printf 'Tests: PASS | install guards\n'; else printf 'PASS install guards: already-installed and not-installed detection\n'; fi