Review: one local name is still a local name; a local name is unique; recovery knows it; recipes read as instructions; a tag before a digest; ask fails at once when nothing serves

A secrets object with one local name delivered no file. Two requirements could share
a local name. secret recover and the export could not tell two locals apart. The
recipe check missed continued lines and read heredoc bodies as bases. repo:tag@digest
kept the tag in the repository. ask now publishes mandatory, so a tool nothing serves
is said at once rather than after the wait.

internal/link/ask.go

@@ -49,9 +49,12 @@ func Ask(ctx context.Context, channel *amqp.Channel, module, tool string, args j
 		return Answer{}, err
 	}
 
+	// Mandatory, so a request nothing consumes comes straight back: a module that is down, or a
+	// tool that does not exist, is said at once rather than after the whole wait.
+	returned := channel.NotifyReturn(make(chan amqp.Return, 1))
 	id := fmt.Sprintf("ask-%d", time.Now().UnixNano())
 	key := module + "." + tool
-	if err := channel.PublishWithContext(ctx, RPCExchange, key, false, false, amqp.Publishing{
+	if err := channel.PublishWithContext(ctx, RPCExchange, key, true, false, amqp.Publishing{
 		ContentType:   "application/json",
 		CorrelationId: id,
 		ReplyTo:       replies.Name,
@@ -64,6 +67,13 @@ func Ask(ctx context.Context, channel *amqp.Channel, module, tool string, args j
 	defer cancel()
 	for {
 		select {
+		case back := <-returned:
+			if back.CorrelationId == id {
+				return Answer{}, fmt.Errorf(
+					"nothing serves %s: no runtime has bound %q on the broker. The module is not "+
+						"assigned, its runtime is not up, or it serves no such tool — `status` "+
+						"says whether the machine carrying it has applied", module, key)
+			}
 		case <-waiting.Done():
 			return Answer{}, fmt.Errorf(
 				"%s did not answer within %s. Its runtime serves %q when it is up and has bound "+