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/channels/plugins/actions/discord/handle-action.ts

@@ -41,6 +41,7 @@ export async function handleDiscordMessageAction(
     const mediaUrl = readStringParam(params, "media", { trim: false });
     const replyTo = readStringParam(params, "replyTo");
     const embeds = Array.isArray(params.embeds) ? params.embeds : undefined;
+    const asVoice = params.asVoice === true;
     return await handleDiscordAction(
       {
         action: "sendMessage",
@@ -50,6 +51,7 @@ export async function handleDiscordMessageAction(
         mediaUrl: mediaUrl ?? undefined,
         replyTo: replyTo ?? undefined,
         embeds,
+        asVoice,
       },
       cfg,
     );