Standardise soft-dependency integrations (PluginNameIntegration) + kill the Dependencies god-class

Replace the Dependencies god-class with a standard integration abstraction:

• integration/Integration interface: pluginName(), hook(Plugin)->boolean, required() (default false).
• One <PluginName>Integration per soft-depend: WorldGuardIntegration, GriefPreventionIntegration, NexoIntegration, and TreasuryIntegration (which absorbs adapters/TreasuryEconomyProvider — the economy is an integration, and required()=true).
• integration/IntegrationRegistrar (@Singleton Listener) replaces Dependencies: hooks every present+enabled integration at enable, hooks late ones on PluginEnableEvent, and fails enable if a required integration is missing. Integrations are collected via a Guice Multibinder<Integration>.
• adapters/TreasuryEconomyProvider deleted (logic → TreasuryIntegration); the WorldGuard onLoad flag registration stays a pre-Guice static call in ChestShop.onLoad.