fix(support): agent-demo self-seeds (join default queue + go online); tickets always get a queue

Root cause of 'waiting for escalations': no queue/membership meant tickets were
never assigned. Now: createTicket find-or-creates a default queue; new joinDefault
endpoint + useGoOnline hook; agent-demo joins+goes-AVAILABLE on load (assignPending
picks up any waiting ticket). smoke-support self-seeds (no seed script needed).
Also: vitest singleFork to end cross-file DB races (45 tests deterministic).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/agent-demo/src/App.tsx

@@ -2,7 +2,7 @@ import { useEffect, useState } from 'react';
 import {
   SupportProvider,
   useAssignedTickets,
-  useAvailability,
+  useGoOnline,
   useThread,
   useMessages,
   type Ticket,
@@ -57,12 +57,12 @@ function AgentChat({ threadId }: { threadId: string }) {
 }
 
 function AgentInner() {
-  const setAvailability = useAvailability();
+  const goOnline = useGoOnline();
   const { tickets } = useAssignedTickets();
   const [threadId, setThreadId] = useState<string | null>(null);
 
   useEffect(() => {
-    void setAvailability('AVAILABLE');
+    void goOnline(); // join default queue + go AVAILABLE
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, []);