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/main.go

@@ -55,6 +55,11 @@ is dialled except the broker.
   MESH_BROKER_FILE   a file the mesh sealed to this machine holding the same
   MESH_REGISTRY      host:port to publish artifacts to, when the mesh has not said
   MESH_BINDING       a file the mesh wrote saying where the artifact store is
+  MESH_PACKAGE_BINDING  a file the mesh wrote saying where the package registry is
+  MESH_NPM_TOKEN_FILE   a file the mesh sealed holding the token for it
+  MESH_NPM_REGISTRY  a package registry URL, when the mesh has not said (a person, the bootstrap)
+  MESH_NPM_TOKEN     the token for it, likewise
+  MESH_NPM_SCOPE     the scope it answers for (default: @novox)
   MESH_WORKSPACE     where to clone and build (default: a temporary directory)
 
 It also builds one module and stops, which is how a mesh is raised — before there is a
@@ -189,11 +194,18 @@ func answer(ctx context.Context, channel *amqp.Channel, publisher builder.Publis
 	}
 	fmt.Fprintln(os.Stderr)
 
-	built, err := builder.Build(ctx, builder.Command, publisher,
-		request.Repository, request.Path, request.Ref, workspace, request.Held,
-		func(step, message string) {
-			fmt.Fprintf(os.Stderr, "  [%s] %s\n", step, message)
-		})
+	npmrc, err := packagesFrom()
+	var built builder.Result
+	if err == nil {
+		// The package-registry credential is a build input, so it is resolved before the clone: a
+		// build that could not have resolved its dependencies is refused in front of the reason,
+		// not after a clone that then fails at npm ci.
+		built, err = builder.Build(ctx, builder.Command, publisher,
+			request.Repository, request.Path, request.Ref, workspace, request.Held, npmrc,
+			func(step, message string) {
+				fmt.Fprintf(os.Stderr, "  [%s] %s\n", step, message)
+			})
+	}
 	if err != nil {
 		// A failure is a result. A build that fails and says nothing is indistinguishable from a
 		// builder that is not running, and those want completely different responses.
@@ -279,6 +291,82 @@ func moduleOf(manifest json.RawMessage) string {
 	return named.Module
 }
 
+// packagesFrom is where a build resolves the mesh's own published packages — the SDK above all
+// (novox/hq ADR 0076, issue 053).
+//
+// Preferably from the mesh: a package-registry binding names the endpoint the way the artifact
+// store's binding does, and a sealed token file the credential the way the broker's does. The
+// environment variables remain for a builder run by a person, and for the bootstrap, where there is
+// no registry yet — there the result is disabled and a build that needs no mesh-published dependency
+// builds anyway.
+func packagesFrom() (builder.Npmrc, error) {
+	scope := strings.TrimSpace(os.Getenv("MESH_NPM_SCOPE"))
+	if scope == "" {
+		scope = "@novox"
+	}
+
+	registry := strings.TrimSpace(os.Getenv("MESH_NPM_REGISTRY"))
+	var username string
+	if path := strings.TrimSpace(os.Getenv("MESH_PACKAGE_BINDING")); path != "" {
+		raw, err := os.ReadFile(path)
+		if err != nil {
+			return builder.Npmrc{}, fmt.Errorf("cannot read what the mesh said about the package registry: %w", err)
+		}
+		var told struct {
+			From   string         `json:"from"`
+			At     string         `json:"at"`
+			As     string         `json:"as"`
+			Serves map[string]any `json:"serves"`
+		}
+		if err := json.Unmarshal(raw, &told); err != nil {
+			return builder.Npmrc{}, fmt.Errorf("%s is not a binding: %w", path, err)
+		}
+		if told.At == "" {
+			return builder.Npmrc{}, fmt.Errorf(
+				"%s says the package registry is on %q and gives no address for it", path, told.From)
+		}
+		// Composed from what the provider serves, so nothing here knows gitea's URL shape from
+		// another registry's: it states its port, the path its registry answers on, and the scheme.
+		scheme := "https"
+		if s, ok := told.Serves["scheme"]; ok {
+			scheme = fmt.Sprintf("%v", s)
+		}
+		port, ok := told.Serves["port"]
+		if !ok {
+			return builder.Npmrc{}, fmt.Errorf("%s says nothing about which port the package registry answers on", path)
+		}
+		npmPath, ok := told.Serves["npm-path"]
+		if !ok {
+			return builder.Npmrc{}, fmt.Errorf("%s says nothing about the path the package registry answers on", path)
+		}
+		registry = fmt.Sprintf("%s://%s:%v%v", scheme, told.At, port, npmPath)
+		username = told.As
+	}
+
+	// The credential the mesh sealed to this machine. The mesh authenticates the ordinary way — a
+	// generated password the provider only applies (novox/hq ADR 0048) — so with a username this is
+	// a password (basic auth); without one it is a bearer token a provider minted.
+	secret := strings.TrimSpace(os.Getenv("MESH_NPM_TOKEN"))
+	if path := strings.TrimSpace(os.Getenv("MESH_NPM_TOKEN_FILE")); path != "" {
+		raw, err := os.ReadFile(path)
+		if err != nil {
+			return builder.Npmrc{}, fmt.Errorf("cannot read this builder's package-registry credential: %w", err)
+		}
+		secret = strings.TrimSpace(string(raw))
+	}
+	if u := strings.TrimSpace(os.Getenv("MESH_NPM_USER")); u != "" {
+		username = u
+	}
+
+	if registry == "" && secret == "" {
+		return builder.Npmrc{}, nil
+	}
+	if username != "" {
+		return builder.Npmrc{Scope: scope, Registry: registry, Username: username, Password: secret}, nil
+	}
+	return builder.Npmrc{Scope: scope, Registry: registry, Token: secret}, nil
+}
+
 func short(commit string) string {
 	if len(commit) > 8 {
 		return commit[:8]