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/discord/send.ts

@@ -37,7 +37,12 @@ export {
   searchMessagesDiscord,
   unpinMessageDiscord,
 } from "./send.messages.js";
-export { sendMessageDiscord, sendPollDiscord, sendStickerDiscord } from "./send.outbound.js";
+export {
+  sendMessageDiscord,
+  sendPollDiscord,
+  sendStickerDiscord,
+  sendVoiceMessageDiscord,
+} from "./send.outbound.js";
 export {
   fetchChannelPermissionsDiscord,
   fetchReactionsDiscord,