feat(discord): add silent support for voice messages

- Add silent flag to sendDiscordVoiceMessage
- Combines VOICE_MESSAGE (8192) + SUPPRESS_NOTIFICATIONS (4096) flags
- Pass silent through VoiceMessageOpts and action handlers

src/agents/tools/discord-actions-messaging.ts

@@ -258,6 +258,7 @@ export async function handleDiscordMessagingAction(
         const result = await sendVoiceMessageDiscord(to, mediaUrl, {
           ...(accountId ? { accountId } : {}),
           replyTo,
+          silent,
         });
         return jsonResult({ ok: true, result, voiceMessage: true });
       }