Move releases to GitHub, with one bridge release on Gitea

GitHub has no generic package registry, so releases become GitHub
release assets. The updater and install.sh read releases/latest/download
and releases/download/<v>/, plain redirects with no API rate limit.
make release publishes a draft with all assets, then makes it latest.
make release-gitea publishes the same version to the old Gitea registry
for binaries that still update from there, and refuses until GitHub
serves it. See D33.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/submarine/main.go

@@ -93,7 +93,7 @@ func main() {
 }
 
 // latestRelease returns the version of a newer release, or "" if there is
-// none or the registry can't be reached within a few seconds.
+// none or GitHub can't be reached within a few seconds.
 func latestRelease(ctx context.Context) string {
 	ctx, cancel := context.WithTimeout(ctx, 3*time.Second)
 	defer cancel()