[raphael]

Replace the fixed sleep 10 in release-goa with a reusable warm-proxy target (also used by release-preview) that:

1. Polls gopkg.in/goadesign/goa.v3 via git ls-remote until the newly pushed tag is visible so proxy.golang.org is never queried before gopkg.in exposes the ref (avoiding negative-cache 404 responses on @v/<tag>.info).
2. Warms proxy.golang.org using the tag's commit hash (@v/<commit>.info) and verifies sum.golang.org/lookup/goa.design/goa/v3@<tag> returns 200 OK.
3. Warms @v/<tag>.info and runs go mod download against proxy.golang.org and sum.golang.org before release-examples and release-plugins start.