refactor(issues): migrate all consumers to TanStack Query (Phase 1, Commits 5-10)

- Migrate issue-detail.tsx: useQuery for issue data, useUpdateIssue/useDeleteIssue
- Migrate issues-page.tsx, my-issues-page.tsx, board-card.tsx: useQuery for list
- Migrate batch-action-toolbar.tsx, create-issue.tsx: mutation hooks
- Migrate edge consumers: mention-suggestion, mention-view, agents page, issue-mention-card
- Remove Zustand writes from WS sync (TQ cache is now sole source of truth)
- Remove useIssueStore.fetch() dependency from workspace store
- Gut useIssueStore to client-only: { activeIssueId, setActiveIssue }
- Update test wrappers with QueryClientProvider

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

apps/web/features/issues/components/board-card.tsx

@@ -8,8 +8,7 @@ import { toast } from "sonner";
 import type { Issue, UpdateIssueRequest } from "@/shared/types";
 import { CalendarDays } from "lucide-react";
 import { ActorAvatar } from "@/components/common/actor-avatar";
-import { api } from "@/shared/api";
-import { useIssueStore } from "@/features/issues/store";
+import { useUpdateIssue } from "@core/issues/mutations";
 import { PriorityIcon } from "./priority-icon";
 import { PriorityPicker, AssigneePicker, DueDatePicker } from "./pickers";
 import { PRIORITY_CONFIG } from "@/features/issues/config";
@@ -46,16 +45,15 @@ export const BoardCardContent = memo(function BoardCardContent({
   const storeProperties = useViewStore((s) => s.cardProperties);
   const priorityCfg = PRIORITY_CONFIG[issue.priority];
 
+  const updateIssueMutation = useUpdateIssue();
   const handleUpdate = useCallback(
     (updates: Partial<UpdateIssueRequest>) => {
-      const prev = { ...issue };
-      useIssueStore.getState().updateIssue(issue.id, updates);
-      api.updateIssue(issue.id, updates).catch(() => {
-        useIssueStore.getState().updateIssue(issue.id, prev);
-        toast.error("Failed to update issue");
-      });
+      updateIssueMutation.mutate(
+        { id: issue.id, ...updates },
+        { onError: () => toast.error("Failed to update issue") },
+      );
     },
-    [issue],
+    [issue.id, updateIssueMutation],
   );
 
   const showPriority = storeProperties.priority;