fix(slack): process thread_broadcast messages instead of dropping them

When Slack users reply to threads with "Also send to channel" enabled,
the message arrives with subtype "thread_broadcast". The handler logged
a system event but returned early without calling handleSlackMessage,
so the bot never responded to these messages.

Now calls handleSlackMessage after the system event so the message
content is processed and the bot can reply.

Fixes #4351

src/slack/monitor/events/messages.ts

@@ -108,6 +108,7 @@ export function registerSlackMessageEvents(params: {
           sessionKey,
           contextKey: `slack:thread:broadcast:${channelId ?? "unknown"}:${messageId ?? "unknown"}`,
         });
+        await handleSlackMessage(message, { source: "message" });
         return;
       }