npc-shops: generic NPC shop installer + mount shop 15 on NPC 20368

tools/server/item-mall-shop becomes tools/server/npc-shops with one
directory per shop (shop.conf + items.tsv) and install.sh <shop dir>.
New mount shop: 35 ride tickets on NPC 20368 (renamed 坐骑商人 via
NPC_NAME -> mob_names.txt) beside the stable boy in a1/b1/c1. Verified
on 203: 35 slots at the drafted prices, bought + used 71114 -> mounted.
zh overlay: 71118 虎坐骑券(黄), 20368 坐骑商人.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

data/zh_40250/item_names_zh_overlay.txt

@@ -1368,6 +1368,7 @@ VNUM	LOCALE_NAME
 71115	野猪坐骑券(蓝)
 71116	狼坐骑券(黄)
 71117	狼坐骑券(蓝)
+71118	虎坐骑券(黄)
 71119	虎坐骑券(蓝)
 71120	狮坐骑券(黄)
 71121	狮坐骑券(蓝)

data/zh_40250/mob_names_zh_overlay.txt

@@ -165,7 +165,7 @@ VNUM	LOCALE_NAME
 20218	战甲母驯鹿
 20219	万圣黑马
 20367	墓穴守卫
-20368	铁算盘
+20368	坐骑商人
 20369	李忠
 20370	中毒的士兵
 20371	古代石碑