fix(slack): prevent duplicate DM processing from app_mention events

Fixes duplicate message processing in Slack DMs where both message.im
and app_mention events fire for the same message, causing:
- 2x token/credit usage per message
- 2x API calls
- Duplicate agent invocations with same runId

Root cause: app_mention events should only fire for channel mentions,
not DMs. Added channel_type check to skip im/mpim in app_mention handler.

Evidence of bug (from production logs):
- Same runId firing twice within 200-300ms
- Example: runId 13cd482c... at 20:32:42.699Z and 20:32:42.954Z

After fix:
- One message = one runId = one processing run
- 50% reduction in duplicate processing

src/slack/monitor/events/messages.ts

@@ -63,6 +63,14 @@ export function registerSlackMessageEvents(params: {
       }
 
       const mention = event as SlackAppMentionEvent;
+
+      // Skip app_mention for DMs - they're already handled by message.im event
+      // This prevents duplicate processing when both message and app_mention fire for DMs
+      const channelType = mention.channel_type;
+      if (channelType === "im" || channelType === "mpim") {
+        return;
+      }
+
       await handleSlackMessage(mention as unknown as SlackMessageEvent, {
         source: "app_mention",
         wasMentioned: true,