CUB-125: address Grimm review — tests, type fixes, error state circuit breaker

- Add missing 'offline' to AgentStatus union type (types/index.ts)
- Add max-retry circuit breaker to useSSE; error state is now reachable
- Wire typed SSE payloads (SSEPayloadMap discriminated union) into useRealtimeSync
- Add Vitest + 20 unit tests: useSSE lifecycle, back-off, circuit breaker,
  event parsing, cleanup; useRealtimeSync event-to-invalidation mapping
- Rebase on dev to remove stale CUB-119 legacy-deletion commit and align
  CI workflow (dev already consolidated into single dev.yml)
- Tests: npm test → 20/20 pass; Build: npm run build → 0 errors

frontend/src/hooks/useRealtimeSync.test.tsx

@@ -0,0 +1,129 @@
+/**
+ * Tests for useRealtimeSync — event → query invalidation mapping.
+ *
+ * Uses .tsx extension so Vite/OXC can parse JSX in the wrapper component.
+ */
+import { renderHook } from '@testing-library/react'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
+import * as useSSEModule from './useSSE'
+import { useRealtimeSync } from './useRealtimeSync'
+import React from 'react'
+import type { SSEMessage } from '../services/sse'
+
+describe('useRealtimeSync', () => {
+  let queryClient: QueryClient
+  let mockSSEOnMessage: ((msg: { type: string; data: unknown }) => void) | null = null
+
+  beforeEach(() => {
+    queryClient = new QueryClient({
+      defaultOptions: { queries: { retry: false } },
+    })
+    mockSSEOnMessage = null
+
+    // Spy on useSSE to capture the onMessage callback
+    vi.spyOn(useSSEModule, 'useSSE').mockImplementation((opts) => {
+      mockSSEOnMessage = opts?.onMessage ?? null
+      return { status: 'connected' }
+    })
+  })
+
+  afterEach(() => {
+    vi.restoreAllMocks()
+  })
+
+  function render() {
+    return renderHook(() => useRealtimeSync(), {
+      wrapper: ({ children }: { children: React.ReactNode }) => (
+        React.createElement(QueryClientProvider, { client: queryClient }, children)
+      ),
+    })
+  }
+
+  it('invalidates ["agents"] on agent.status event', async () => {
+    const invalidateSpy = vi.spyOn(queryClient, 'invalidateQueries')
+    render()
+
+    const msg: SSEMessage = {
+      type: 'agent.status',
+      data: { agentId: 'a1', status: 'active' },
+    }
+    mockSSEOnMessage!(msg)
+
+    expect(invalidateSpy).toHaveBeenCalledWith({ queryKey: ['agents'] })
+    expect(invalidateSpy).toHaveBeenCalledTimes(1)
+  })
+
+  it('invalidates ["tasks"] and ["agents"] on agent.task event', async () => {
+    const invalidateSpy = vi.spyOn(queryClient, 'invalidateQueries')
+    render()
+
+    const msg: SSEMessage = {
+      type: 'agent.task',
+      data: { agentId: 'a1', taskId: 't1', title: 'Test', action: 'assigned' },
+    }
+    mockSSEOnMessage!(msg)
+
+    expect(invalidateSpy).toHaveBeenCalledWith({ queryKey: ['tasks'] })
+    expect(invalidateSpy).toHaveBeenCalledWith({ queryKey: ['agents'] })
+    expect(invalidateSpy).toHaveBeenCalledTimes(2)
+  })
+
+  it('invalidates ["tasks"] and ["agents"] on agent.progress event', async () => {
+    const invalidateSpy = vi.spyOn(queryClient, 'invalidateQueries')
+    render()
+
+    const msg: SSEMessage = {
+      type: 'agent.progress',
+      data: { agentId: 'a1', taskId: 't1', progress: 50, message: 'working' },
+    }
+    mockSSEOnMessage!(msg)
+
+    expect(invalidateSpy).toHaveBeenCalledWith({ queryKey: ['tasks'] })
+    expect(invalidateSpy).toHaveBeenCalledWith({ queryKey: ['agents'] })
+    expect(invalidateSpy).toHaveBeenCalledTimes(2)
+  })
+
+  it('invalidates ["agents"], ["sessions"], ["tasks"] on fleet.update event', async () => {
+    const invalidateSpy = vi.spyOn(queryClient, 'invalidateQueries')
+    render()
+
+    const msg: SSEMessage = {
+      type: 'fleet.update',
+      data: { timestamp: '2026-05-20T12:00:00Z', agentCount: 5 },
+    }
+    mockSSEOnMessage!(msg)
+
+    expect(invalidateSpy).toHaveBeenCalledWith({ queryKey: ['agents'] })
+    expect(invalidateSpy).toHaveBeenCalledWith({ queryKey: ['sessions'] })
+    expect(invalidateSpy).toHaveBeenCalledWith({ queryKey: ['tasks'] })
+    expect(invalidateSpy).toHaveBeenCalledTimes(3)
+  })
+
+  it('does nothing on connected event', async () => {
+    const invalidateSpy = vi.spyOn(queryClient, 'invalidateQueries')
+    render()
+
+    const msg: SSEMessage = {
+      type: 'connected',
+      data: { clientCount: 1 },
+    }
+    mockSSEOnMessage!(msg)
+
+    expect(invalidateSpy).not.toHaveBeenCalled()
+  })
+
+  it('does nothing on unknown event types', async () => {
+    const invalidateSpy = vi.spyOn(queryClient, 'invalidateQueries')
+    render()
+
+    mockSSEOnMessage!({ type: 'unknown.event', data: {} })
+
+    expect(invalidateSpy).not.toHaveBeenCalled()
+  })
+
+  it('returns sseStatus from useSSE', () => {
+    const { result } = render()
+    expect(result.current.sseStatus).toBe('connected')
+  })
+})