Make the login shell's execute optional, so a machine may withhold it

execute runs any command as the operator account, which can become root
without a person. The operator withholds it on the control-node until a
call needs a person's approval (hq ADR 0268); the holder withholds it per
machine through its own setting. With execute required, that holder could
not hold the seat there and would be judged silent. ADR 0246's optional
mark lets it hold the seat without serving the verb.

cmd/mesh-controller/holders_silent_test.go

@@ -92,3 +92,26 @@ func TestHoldingNeedsAnAnswer(t *testing.T) {
 		}
 	}
 }
+
+// The control-node withholds the login shell's `execute` (novox/hq ADR 0268): its holder there serves
+// nothing on the seat, and must not be judged silent for it, as the store's rows read it back.
+func TestALoginShellWithholdingExecuteIsNotSilent(t *testing.T) {
+	defer catalogue.UseSeats(catalogue.DefaultSeats())
+	var rows []catalogue.Seat
+	for _, s := range catalogue.DefaultSeats() {
+		stored := s
+		stored.Serves = nil
+		for _, v := range s.Serves {
+			v.Optional = false // the store never keeps the mark
+			stored.Serves = append(stored.Serves, v)
+		}
+		rows = append(rows, stored)
+	}
+	catalogue.UseSeats(rows)
+	recorded := []catalogue.Held{{Claim: catalogue.LoginShellSeat, Scope: catalogue.ScopeNode, Node: "anchor", Module: "zsh"}}
+	expected := holdersToHear(catalogue.SeatsWithAProtocol(), recorded, nil, map[string]bool{"anchor": true}, nil, time.Now())
+	if _, asked := expected[catalogue.LoginShellSeat]; asked {
+		t.Fatalf("the login shell's holder is expected to answer, so withholding execute would be said silent: %v",
+			expected[catalogue.LoginShellSeat])
+	}
+}