Enhance command registration process and improve guild command handling

- Introduced a new `buildCommandBody` function to prevent duplicate command definitions during registration.
- Implemented `clearGuildCommandLeftovers` to remove unused guild-scoped commands, ensuring a clean command picker in Discord.
- Updated command registration logic to clear global commands when using guild-specific commands, improving command visibility and user experience.
- Enhanced logging to provide clearer insights into command registration and cleanup processes.

.env.example

@@ -4,6 +4,10 @@ NODE_ENV=production
 BOT_TOKEN=
 BOT_CLIENT_ID=
 BOT_CLIENT_SECRET=
+# Optional: register slash commands only on this guild (instant sync for dev).
+# Leave empty in production so commands are global. When set, global commands
+# are cleared; when empty, leftover guild-scoped commands are cleared on startup
+# so Discord does not show every command twice.
 BOT_GUILD_ID=
 DATABASE_URL=postgresql://nexumi:nexumi@postgres:5432/nexumi
 REDIS_URL=redis://redis:6379