fix(agent-runner): remove thread_id filter and fix processing ack on empty result

The concurrent poll in processQuery filtered out messages with
mismatched thread_ids, causing a deadlock when the initial batch
(e.g. a host-generated welcome trigger with null thread_id) completed
but follow-ups arrived with a different thread_id (e.g. a Discord DM).
The query stayed open waiting for matching-thread pushes that never
came, blocking the poll loop indefinitely.

Thread routing is the router's concern — per-thread sessions already
isolate threads into separate containers; shared sessions intentionally
merge everything. Removed the filter.

Also fixed processing_ack: a result event (with or without text) means
the turn is done, but markCompleted only ran when event.text was truthy.
When the agent responded via MCP send_message (empty result text), the
initial batch stayed in 'processing' for the query's lifetime, creating
false stuck signals in the host sweep. Now marks completed on any result
event.

Belt-and-suspenders: init-first-agent welcome trigger now sets threadId
to the DM platform_id instead of null.

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

scripts/init-first-agent.ts

@@ -363,7 +363,7 @@ async function sendWelcomeViaCliSocket(
           to: {
             channelType: dmMg.channel_type,
             platformId: dmMg.platform_id,
-            threadId: null,
+            threadId: dmMg.platform_id,
           },
         }) + '\n';
       socket.write(payload, (err) => {