The builder resolves the SDK from the mesh registry, and can publish packages

A new 'package' artifact kind builds a module's own code on a public base image
and publishes it to the mesh's package registry by version (hq ADR 0076) — the
SDK above all, which the toolchain is built from and so cannot be built in the
toolchain. The credential a build needs to resolve or publish packages is
rendered as an .npmrc (basic auth, hq ADR 0048) and given to an image build as a
buildkit secret, never a layer, so a token is not baked into the toolchain image.

Claude-Session: https://claude.ai/code/session_01D6qtiYU3P9jk3pnAXyAFyx

cmd/mesh-builder/once.go

@@ -84,7 +84,11 @@ func buildOnce(ctx context.Context, args []string) error {
 	}
 	fmt.Fprintln(os.Stderr)
 
-	built, buildErr := builder.Build(ctx, builder.Command, publisher, repository, *path, *ref, where, bases,
+	npmrc, err := packagesFrom()
+	if err != nil {
+		return err
+	}
+	built, buildErr := builder.Build(ctx, builder.Command, publisher, repository, *path, *ref, where, bases, npmrc,
 		func(step, message string) { fmt.Fprintf(os.Stderr, "  [%s] %s\n", step, message) })
 	if buildErr != nil {
 		return buildErr