feat(slack): include thread metadata (thread_ts, parent_user_id) in agent context

Adds thread_ts and parent_user_id to the Slack message footer for thread
replies, giving agents awareness of thread context. Top-level messages
remain unchanged.

Includes tests verifying:
- Thread replies include thread_ts and parent_user_id in footer
- Top-level messages exclude thread metadata

src/slack/monitor/message-handler/prepare.ts

@@ -399,7 +399,10 @@ export async function prepareSlackMessage(params: {
       GroupSubject: isRoomish ? roomLabel : undefined,
       From: slackFrom,
     }) ?? (isDirectMessage ? senderName : roomLabel);
-  const textWithId = `${rawBody}\n[slack message id: ${message.ts} channel: ${message.channel}]`;
+  const threadInfo = message.thread_ts
+    ? ` thread_ts: ${message.thread_ts}${message.parent_user_id ? ` parent_user: ${message.parent_user_id}` : ""}`
+    : "";
+  const textWithId = `${rawBody}\n[slack message id: ${message.ts} channel: ${message.channel}${threadInfo}]`;
   const storePath = resolveStorePath(ctx.cfg.session?.store, {
     agentId: route.agentId,
   });