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

Dockerfile

@@ -51,7 +51,7 @@ RUN go mod download
 # the lint stage above.
 COPY . .
 
-# Run tests and build. Both first run `make assets`, which extracts Alpine.js
+# Run tests and build. Both first run script/assets, which extracts Alpine.js
 # from its tarball in 3p/.
 RUN make test