Add prefix command support

[Ntalcme]

Goal: Support message-based prefix commands alongside slash commands.

Scope:

• Add PrefixCommand type guard (isPrefixCommand)
• Add prefix loader via loadModules
• Add a prefix registry (shared module)
• Handle messageCreate event: parse prefix, route to command
• Enable MessageContent privileged intent
• Add !ping as the first prefix command

Notes: MessageContent is a privileged intent — must be enabled in the Discord developer portal.