fix: mention closure, onSettled invalidation, cleanup singleton

- Fix Tiptap mention: pass QueryClient via closure from ContentEditor
  instead of getQueryClient() singleton (resolves @mention empty list)
- Add onSettled invalidation to useUpdateIssue (prevents cache drift
  with staleTime: Infinity + self-event WS filter)
- Add cache shape comment to issueListOptions (select transforms
  ListIssuesResponse → Issue[], but cache stores raw response)
- Memoize sidebar inbox dedup computation
- Remove dead getQueryClient/setQueryClient singleton + window property
- Remove ActorSync component and _members/_agents Zustand mirror
  (superseded by closure approach)

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

apps/web/core/query-client.ts

@@ -1,7 +1,5 @@
 import { QueryClient } from "@tanstack/react-query";
 
-let _queryClient: QueryClient | null = null;
-
 export function createQueryClient(): QueryClient {
   return new QueryClient({
     defaultOptions: {
@@ -18,14 +16,3 @@ export function createQueryClient(): QueryClient {
     },
   });
 }
-
-/** Called by QueryProvider on mount to register the singleton. */
-export function setQueryClient(client: QueryClient) {
-  _queryClient = client;
-}
-
-/** Access QueryClient outside React tree (WS handlers, Zustand actions). */
-export function getQueryClient(): QueryClient {
-  if (!_queryClient) throw new Error("QueryClient not initialized");
-  return _queryClient;
-}