builder: a clone may offer the forge's credential, through git's own store

A private repository could not be built: the builder clones anonymously,
and had no way to say who it is. It already holds exactly one credential
to exactly the right place — the package-registry binding and its sealed
secret, one gitea user whose password answers npm and git alike — so a
clone now offers that, and nothing new is minted or carried.

Offered, never pushed: the credential is written as a git
credential-store file (0600, in the workspace, never argv) and named
with -c credential.helper, so git itself decides when it applies — only
on an authentication challenge, and only for the URL it was written
for, scheme, host and port included. A public repository clones exactly
as before; a repository on any other host is never shown it. The same
store rides along on an artifact's own context clone, so a private
module with a private context builds too.

cmd/mesh-builder/main.go

@@ -24,6 +24,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"net/url"
 	"os"
 	"os/signal"
 	"strings"
@@ -202,6 +203,7 @@ func answer(ctx context.Context, channel *amqp.Channel, publisher builder.Publis
 		// 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,
+			forgeFrom(),
 			func(step, message string) {
 				fmt.Fprintf(os.Stderr, "  [%s] %s\n", step, message)
 			})
@@ -367,6 +369,53 @@ func packagesFrom() (builder.Npmrc, error) {
 	return builder.Npmrc{Scope: scope, Registry: registry, Token: secret}, nil
 }
 
+// forgeFrom is the git credential this builder may offer a clone, composed from the same binding
+// and sealed secret its package-registry half already reads: the forge that answers npm is the
+// forge that hosts the repositories, and its provisioner applies one password to one user for
+// both. Anything missing means no credential, and every clone stays anonymous — which is all a
+// mesh of public repositories ever needs.
+//
+// The URL names the binding's own address — the machine the mesh says the forge is on — so a
+// private repository is registered and built by that address, and a clone of anything else is
+// never shown this credential (git's credential store matches the whole origin).
+func forgeFrom() builder.GitCredential {
+	path := strings.TrimSpace(os.Getenv("MESH_PACKAGE_BINDING"))
+	if path == "" {
+		return builder.GitCredential{}
+	}
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		return builder.GitCredential{}
+	}
+	var told struct {
+		At     string         `json:"at"`
+		As     string         `json:"as"`
+		Serves map[string]any `json:"serves"`
+	}
+	if err := json.Unmarshal(raw, &told); err != nil || told.At == "" || told.As == "" {
+		return builder.GitCredential{}
+	}
+	secret := strings.TrimSpace(os.Getenv("MESH_NPM_TOKEN"))
+	if file := strings.TrimSpace(os.Getenv("MESH_NPM_TOKEN_FILE")); file != "" {
+		if raw, err := os.ReadFile(file); err == nil {
+			secret = strings.TrimSpace(string(raw))
+		}
+	}
+	if secret == "" {
+		return builder.GitCredential{}
+	}
+	scheme := "https"
+	if s, ok := told.Serves["scheme"]; ok {
+		scheme = fmt.Sprintf("%v", s)
+	}
+	host := told.At
+	if port, ok := told.Serves["port"]; ok {
+		host = fmt.Sprintf("%s:%v", told.At, port)
+	}
+	made := url.URL{Scheme: scheme, User: url.UserPassword(told.As, secret), Host: host}
+	return builder.GitCredential{URL: made.String()}
+}
+
 func short(commit string) string {
 	if len(commit) > 8 {
 		return commit[:8]