Change ChestShop business alias sign prefix from B:@ to A:

Business shop aliases are currently written on the owner line as B:@alias. Change the sign form to A:alias (letters only, 1-12 chars, so A: + alias fits the 15-char line).

• B:@ is replaced outright (never shipped to prod). Existing B:@ signs on the dev clone stop resolving.
• B:<base36> and legacy B:<firm name> are unchanged.
• Business alias messages, Explorer docs and wiki updated to show A:.