feat(discord): add voice message support

Adds support for sending Discord voice messages via the message tool
with asVoice: true parameter.

Voice messages require:
- OGG/Opus format (auto-converted if needed via ffmpeg)
- Waveform data (generated from audio samples)
- Duration in seconds
- Message flag 8192 (IS_VOICE_MESSAGE)

Implementation:
- New voice-message.ts with audio processing utilities
- getAudioDuration() using ffprobe
- generateWaveform() samples audio and creates base64 waveform
- ensureOggOpus() converts audio to required format
- sendDiscordVoiceMessage() handles 3-step Discord upload process

Usage:
message(action='send', channel='discord', target='...',
        path='/path/to/audio.mp3', asVoice=true)

Note: Voice messages cannot include text content (Discord limitation)

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

@@ -18,6 +18,7 @@ import {
   sendMessageDiscord,
   sendPollDiscord,
   sendStickerDiscord,
+  sendVoiceMessageDiscord,
   unpinMessageDiscord,
 } from "../../discord/send.js";
 import { resolveDiscordChannelId } from "../../discord/targets.js";
@@ -230,11 +231,25 @@ export async function handleDiscordMessagingAction(
       const to = readStringParam(params, "to", { required: true });
       const content = readStringParam(params, "content", {
         required: true,
+        allowEmpty: true,
       });
       const mediaUrl = readStringParam(params, "mediaUrl");
       const replyTo = readStringParam(params, "replyTo");
+      const asVoice = params.asVoice === true;
       const embeds =
         Array.isArray(params.embeds) && params.embeds.length > 0 ? params.embeds : undefined;
+
+      // Handle voice message sending
+      if (asVoice && mediaUrl) {
+        // Voice messages require a local file path or downloadable URL
+        // They cannot include text content (Discord limitation)
+        const result = await sendVoiceMessageDiscord(to, mediaUrl, {
+          ...(accountId ? { accountId } : {}),
+          replyTo,
+        });
+        return jsonResult({ ok: true, result, voiceMessage: true });
+      }
+
       const result = await sendMessageDiscord(to, content, {
         ...(accountId ? { accountId } : {}),
         mediaUrl,