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

Makefile

@@ -27,13 +27,10 @@ bootstrap:
 setup:
 	@script/setup
 
-# Alpine.js is committed as its npm package tarball in 3p/. This extracts
-# the browser build from it to where go:embed reads it; the extracted file
-# is not committed.
 assets:
-	tar -xzOf 3p/alpinejs-3.14.9.tgz package/dist/cdn.min.js >static/js/alpine.min.js
+	@script/assets
 
-test: assets
+test:
 	@script/test
 
 lint:
@@ -45,7 +42,7 @@ fmt:
 fmt-check:
 	@script/fmt-check
 
-check: assets
+check:
 	@script/check
 
 build: assets