fix: skip empty WhatsApp protocol messages

WhatsApp protocol messages (encryption key distribution, read receipts,
ephemeral settings, senderKeyDistributionMessage) have a message envelope
but no text content. These were being stored in messages.db and processed
by the agent, causing:

1. Agent responds to empty messages, wasting API tokens
2. Container stays alive indefinitely (idle timer resets)
3. Scheduled tasks blocked (queue slot occupied)

This fix skips messages with empty content before calling onMessage,
preventing protocol messages from being stored and processed.

Fixes #250

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/channels/whatsapp.ts

@@ -170,6 +170,10 @@ export class WhatsAppChannel implements Channel {
             msg.message?.imageMessage?.caption ||
             msg.message?.videoMessage?.caption ||
             '';
+
+          // Skip protocol messages with no text content (encryption keys, read receipts, etc.)
+          if (!content) continue;
+
           const sender = msg.key.participant || msg.key.remoteJid || '';
           const senderName = msg.pushName || sender.split('@')[0];