fix: resolve WorthPacketListener compilation failure due to field shadowing in anonymous class

[Copilot]

CI was failing to compile because inside the anonymous PacketAdapter subclass, the unqualified plugin reference resolved to PacketAdapter's own plugin field (type org.bukkit.plugin.Plugin) rather than WorthPacketListener's field (type SellPlugin), making getConfigManager() unavailable.

Change

• WorthPacketListener.java: Qualify plugin as WorthPacketListener.this.plugin inside the anonymous PacketAdapter to explicitly target the outer class's SellPlugin field.

// Before — resolves to PacketAdapter.plugin (type Plugin), no getConfigManager()
if (!plugin.getConfigManager().isWorthEnabled()) return;

// After — resolves to WorthPacketListener.plugin (type SellPlugin)
if (!WorthPacketListener.this.plugin.getConfigManager().isWorthEnabled()) return;