One bus: the AMQP transport is gone from the controller

The mesh runs on the seat's bus alone (novox/hq ADR 0131, design 28 task 5.5). The old
transport's consume loop, build request, tool ask, management API and account scoping are
deleted, and the bus switch with them; the controller connects to the broker seat and to
nothing else. The store-window tests keep their assertions on a bus-less fake, and the tests
that only made sense for the old transport's in-memory holding go with it.

internal/broker/onnats.go

@@ -68,24 +68,16 @@ func BareAddress(address string) string {
 	return "nats://" + bare
 }
 
-// MustBeOneBus refuses a configuration that names both buses for the mesh's own traffic.
-//
-// **Both clients ship and that is the point; both being live is not.** The rollout moves every node
-// at once (ADR 0116 step 5): a mesh half on each is one where a declaration goes out on one bus and
-// the report comes back on the other, and nothing anywhere says so — every component would log
-// success. Refused at start, where it can be said in one sentence.
-func MustBeOneBus(amqp, nats string) error {
-	if strings.TrimSpace(amqp) != "" && strings.TrimSpace(nats) != "" {
-		return fmt.Errorf(
-			"this control plane is told about both buses (%s and %s) and can only be on one. A mesh "+
-				"half on each is one where a declaration goes out on one and the report comes back "+
-				"on the other, and every component reports success while it happens. The rollout "+
-				"moves every node at once: unset %s to stay, or unset %s to move",
-			AMQPVarName, NATSVar, NATSVar, AMQPVarName)
+// BusAddress is where the mesh's bus is, with this process's credential, and refuses to be empty:
+// there is one bus, and a control plane without it can hold records and answer nothing (novox/hq
+// ADR 0131, design 28 task 5.5).
+func BusAddress() (string, error) {
+	address, _, err := OnNATS()
+	if err != nil {
+		return "", err
 	}
-	return nil
+	if address == "" {
+		return "", fmt.Errorf("this control plane has no %s, so it cannot reach the mesh's bus", NATSVar)
+	}
+	return address, nil
 }
-
-// AMQPVarName is the variable naming the bus the mesh runs on today. Named here rather than
-// imported from the link package, for the one direction of dependency.
-const AMQPVarName = "MESH_BROKER_AMQP"