Trust the vault's key alone and write the grants part's user list alone

The controller ended by its unassignment and its lease is never handed, so
the engine's lease gate kept trusting the controller's key and the bus node
kept the controller's list as a fallback nobody writes (issue 1126, A6).
The engine now trusts no controller signature, its hand-over line names the
node seat's hand-over, and the user-list follower writes only the grants
part's list: a key unreadable or deleted keeps the list loaded, and with no
key heard nothing is written. Compositions in the node's bucket name the
file's place alone. The guard no longer asks for the controller's user.

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

cmd/mesh-host/nodeagent_integration_test.go

@@ -160,7 +160,7 @@ func (b *itBed) start(trustOperator bool) (stop func()) {
 	go func() {
 		defer close(done)
 		controller := b.controll.Public().(ed25519.PublicKey)
-		nodeAgent(ctx, b.opts, controller, newMeshTrust(controller), b.session, nil, b.say, func() {},
+		nodeAgent(ctx, b.opts, controller, newMeshTrust(), b.session, nil, b.say, func() {},
 			&agentNow{}, func(context.Context, *agent.Agent) {})
 	}()
 	return func() { cancel(); <-done }