fix: reply in the Slack DM thread the user wrote in

- chat-sdk-bridge: forward thread.id to the router for DMs so sub-thread
  context survives into delivery. Previously hardcoded to null, which
  collapsed every reply to the DM top level.
- router: when a DM (is_group=0) is wired as `shared`, don't auto-escalate
  to per-thread — keep one session for the whole DM and let thread_id
  flow through to the adapter.
- agent-runner poll-loop: defer follow-up messages whose thread_id
  differs from the active turn's routing. Mixing threads into one
  streaming turn sent every reply to the first thread because routing
  is captured at turn start.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

container/agent-runner/src/poll-loop.ts

@@ -249,13 +249,18 @@ async function processQuery(query: AgentQuery, routing: RoutingContext, config:
   const pollHandle = setInterval(() => {
     if (done) return;
 
-    // Skip system messages (MCP tool responses) and admin commands (need fresh query)
+    // Skip system messages (MCP tool responses) and admin commands (need fresh query).
+    // Also defer messages whose thread_id differs from the active turn's routing
+    // — mixing threads into one streaming turn would send the reply to the wrong
+    // thread because `routing` is captured at turn start. The next turn will pick
+    // them up with fresh routing.
     const newMessages = getPendingMessages().filter((m) => {
       if (m.kind === 'system') return false;
       if (m.kind === 'chat' || m.kind === 'chat-sdk') {
         const cmd = categorizeMessage(m);
         if (cmd.category === 'admin') return false;
       }
+      if ((m.thread_id ?? null) !== (routing.threadId ?? null)) return false;
       return true;
     });
     if (newMessages.length > 0) {