Extract Alpine.js in script/test so the pre-commit hook works

The extraction moves from the Makefile's assets target into
script/assets. script/test runs it before the tests, so make test,
make check, script/check and the pre-commit hook all get the file on a
fresh clone; make assets, make build and make dev call the same script.
The README now says only that nothing downloads Alpine.js.

Model: opus-5-5

script/test

@@ -28,6 +28,7 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
+    "$ROOT/script/assets"
     go test -v -race -timeout 90s ./...
 }