fix(discord): autoThread race condition when multiple agents mentioned

When multiple agents with autoThread:true are @mentioned in the same
message, only the first agent successfully creates a thread. Subsequent
agents fail because Discord only allows one thread per message.

Previously, the failure was silently caught and the agent would fall
back to replying in the parent channel.

Now, when thread creation fails, the code re-fetches the message and
checks for an existing thread (created by another agent). If found,
the agent replies in that thread instead of falling back.

Fixes #7508

src/discord/monitor/threading.ts

@@ -358,8 +358,24 @@ export async function maybeCreateDiscordAutoThread(params: {
     return createdId || undefined;
   } catch (err) {
     logVerbose(
-      `discord: autoThread failed for ${params.message.channelId}/${params.message.id}: ${String(err)}`,
+      `discord: autoThread creation failed for ${params.message.channelId}/${params.message.id}: ${String(err)}`,
     );
+    // Race condition: another agent may have already created a thread on this
+    // message. Re-fetch the message to check for an existing thread.
+    try {
+      const msg = (await params.client.rest.get(
+        Routes.channelMessage(params.message.channelId, params.message.id),
+      )) as { thread?: { id?: string } };
+      const existingThreadId = msg?.thread?.id ? String(msg.thread.id) : "";
+      if (existingThreadId) {
+        logVerbose(
+          `discord: autoThread reusing existing thread ${existingThreadId} on ${params.message.channelId}/${params.message.id}`,
+        );
+        return existingThreadId;
+      }
+    } catch {
+      // If the refetch also fails, fall through to return undefined.
+    }
     return undefined;
   }
 }