Rename item-subsystem methods (refactor step 5, items)

getItem→promptPackItem now returns (obj, ok) instead of a nil-signaling
pointer; invName→inventoryName; doPot→applyPotionFuse. C breadcrumbs
kept. Behavior unchanged; suite green.

game/sticks.go

@@ -14,8 +14,8 @@ const (
 func (g *RogueGame) doZap() {
 	p := &g.Player
 
-	obj := g.getItem("zap with", KindWand)
-	if obj == nil {
+	obj, ok := g.promptPackItem("zap with", KindWand)
+	if !ok {
 		return
 	}