fix: replace deprecated discord.js 'ready' event with 'clientReady' (#178)

The 'ready' event is deprecated in discord.js v14 and will break in v15.
Also switch from on() to once() since this event fires only once per login.

Reported-by: Aeo

Written by Cameron ◯ Letta Code

"The quietest fix is the one that prevents the loudest crash." -- a wise linter

src/channels/discord.ts

@@ -132,7 +132,7 @@ Ask the bot owner to approve with:
       partials: [Partials.Channel, Partials.Message, Partials.Reaction, Partials.User],
     });
 
-    this.client.on('ready', () => {
+    this.client.once('clientReady', () => {
       const tag = this.client?.user?.tag || '(unknown)';
       console.log(`[Discord] Bot logged in as ${tag}`);
       console.log(`[Discord] DM policy: ${this.config.dmPolicy}`);