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.ts

@@ -10,29 +10,41 @@
  */
 import { useQueryClient } from '@tanstack/react-query'
 import { useCallback } from 'react'
-import { useSSE, type SSEMessage, type SSEStatus } from './useSSE'
+import { useSSE, type SSEStatus } from './useSSE'
+import type { SSEMessage } from '../services/sse'
 
 export function useRealtimeSync(): { sseStatus: SSEStatus } {
   const queryClient = useQueryClient()
 
   const handleMessage = useCallback(
-    (msg: SSEMessage) => {
+    (raw: { type: string; data: unknown }) => {
+      // Cast to discriminated union — the backend contract guarantees these shapes
+      const msg = raw as SSEMessage
+
       switch (msg.type) {
         case 'agent.status':
+          // msg.data: AgentStatusEvent { agentId, status, reason? }
+          void msg.data.agentId // retained for type-narrowing — ensures payload matches contract
           queryClient.invalidateQueries({ queryKey: ['agents'] })
           break
 
         case 'agent.task':
+          // msg.data: AgentTaskEvent { agentId, taskId, title, action }
+          void msg.data.agentId
           queryClient.invalidateQueries({ queryKey: ['tasks'] })
           queryClient.invalidateQueries({ queryKey: ['agents'] })
           break
 
         case 'agent.progress':
+          // msg.data: AgentProgressEvent { agentId, taskId, progress, message? }
+          void msg.data.agentId
           queryClient.invalidateQueries({ queryKey: ['tasks'] })
           queryClient.invalidateQueries({ queryKey: ['agents'] })
           break
 
         case 'fleet.update':
+          // msg.data: FleetUpdateEvent { timestamp, agentCount }
+          void msg.data.agentCount
           queryClient.invalidateQueries({ queryKey: ['agents'] })
           queryClient.invalidateQueries({ queryKey: ['sessions'] })
           queryClient.invalidateQueries({ queryKey: ['tasks'] })