feat(commands): add beta feature flag support to command system

- Add beta and featureFlag properties to Command interface
- Add beta access check in CommandHandler before command execution
- Show beta feature message to non-whitelisted users

shared/lib/types.ts

@@ -7,6 +7,8 @@ export interface Command {
     execute: (interaction: ChatInputCommandInteraction) => Promise<void> | void;
     autocomplete?: (interaction: AutocompleteInteraction) => Promise<void> | void;
     category?: string;
+    beta?: boolean;
+    featureFlag?: string;
 }
 
 export interface Event<K extends keyof ClientEvents> {