Fix shulker box selling: purple variant undetected, contents wiped on deposit failure

[Copilot]

Two bugs prevented shulker box selling from working correctly: the default purple SHULKER_BOX material was never recognised as a shulker, and contents were stripped from the box before the economy deposit — meaning a failed deposit returned an empty box with no payment.

Changes

• SellManager.isShulkerBox — extend match to cover SHULKER_BOX in addition to the 16 colour-prefixed variants (*_SHULKER_BOX):

  // Before
  return item != null && item.getType().name().endsWith("_SHULKER_BOX");
  
  // After
  String name = item.getType().name();
  return name.equals("SHULKER_BOX") || name.endsWith("_SHULKER_BOX");

• GUIListener.onClose — transactional shulker return

  • Clone the shulker before calling sellShulkerContents.
  • Defer returnItem until after the deposit resolves.
  • On success → return the modified shulker (sold items removed).
  • On failure → return the original clone (contents intact, no item loss).

• README.md — added; covers features, requirements, install, commands, permissions, config reference, shulker box behaviour, and build instructions.