Fix Multiverse / late-loaded world template fill-ins

[Geph]

Summary

Template prompt fill-ins resolve worlds with Bukkit.getWorld(name) at plugin enable / template load. Multiverse worlds that load later (or worlds not present at startup) can end up with empty fill-in option lists.

Suggested approach

• Resolve worlds by name at selection time, not only at load
• Listen for world-load events and refresh template caches
• Support a default / * fill-in bucket so non-world maps still get options

Impact

Classroom servers using Multiverse often see missing or empty template responses in non-default worlds.