docs(20-02): create summary and update state for frontend subscriptions

Phase 20 (Real-Time Subscriptions) is now complete. Both plans delivered:
- 20-01: SSE backend with subscription procedures
- 20-02: Frontend splitLink + subscription hooks on all 3 UI pages

.planning/phases/20-real-time-subscriptions/20-02-SUMMARY.md

@@ -0,0 +1,112 @@
+---
+phase: 20-real-time-subscriptions
+plan: 02
+subsystem: frontend
+tags: [trpc, sse, subscriptions, react-query, live-updates, splitLink]
+
+requires:
+  - phase: 20-real-time-subscriptions
+    plan: 01
+    provides: SSE subscription procedures (onEvent, onAgentUpdate, onTaskUpdate)
+  - phase: 16-frontend-scaffold
+    provides: tRPC client and React Query setup
+
+provides:
+  - splitLink routing subscriptions to httpSubscriptionLink (SSE)
+  - Live update hooks on all three UI pages (dashboard, detail, inbox)
+  - Automatic React Query cache invalidation on domain events
+
+affects: [21-frontend-polish]
+
+tech-stack:
+  added: []
+  patterns: [splitLink for operation-type routing, subscription-driven cache invalidation]
+
+key-files:
+  modified:
+    - packages/web/src/lib/trpc.ts
+    - packages/web/src/routes/initiatives/index.tsx
+    - packages/web/src/routes/initiatives/$id.tsx
+    - packages/web/src/routes/inbox.tsx
+
+key-decisions:
+  - "httpSubscriptionLink (stable export) over unstable_ prefix — both available in @trpc/client v11"
+  - "Subscription-driven invalidation (not local state) — simplest approach, reuses existing React Query cache"
+  - "Silent onError callbacks — pages degrade to manual refresh when backend is not running"
+  - "No new packages needed — splitLink and httpSubscriptionLink ship with @trpc/client v11"
+
+patterns-established:
+  - "useSubscription + useUtils().invalidate for live updates on any page"
+
+duration: 3min
+completed: 2026-02-04
+---
+
+# Plan 20-02: Frontend SSE Subscription Hooks Summary
+
+**Wire frontend tRPC client with SSE subscriptions and live update hooks on all three UI pages**
+
+## Performance
+
+- **Duration:** 3 min
+- **Started:** 2026-02-04
+- **Completed:** 2026-02-04
+- **Tasks:** 2
+- **Files modified:** 4
+
+## Accomplishments
+- Replaced single httpBatchLink with splitLink routing subscriptions to httpSubscriptionLink (SSE)
+- Added onTaskUpdate subscription to dashboard page (invalidates listInitiatives + listPhases)
+- Added onTaskUpdate + onAgentUpdate subscriptions to initiative detail page (invalidates phases, tasks, plans, agents)
+- Added onAgentUpdate subscription to inbox page (invalidates listWaitingAgents + listMessages)
+
+## Task Commits
+
+Each task was committed atomically:
+
+1. **Task 1: Update tRPC client with splitLink for SSE subscriptions** - `eaf3f10` (feat)
+2. **Task 2: Add live update hooks to all three UI pages** - `170ac55` (feat)
+
+## Files Modified
+- `packages/web/src/lib/trpc.ts` - splitLink routing subscriptions to httpSubscriptionLink
+- `packages/web/src/routes/initiatives/index.tsx` - onTaskUpdate subscription for dashboard
+- `packages/web/src/routes/initiatives/$id.tsx` - onTaskUpdate + onAgentUpdate subscriptions for detail
+- `packages/web/src/routes/inbox.tsx` - onAgentUpdate subscription for inbox
+
+## Decisions Made
+- 20-02: httpSubscriptionLink (stable export) used over unstable_ prefix variant
+- 20-02: Subscription-driven invalidation pattern — no local event state, just invalidate React Query caches
+- 20-02: Silent onError callbacks so pages degrade gracefully without backend
+- 20-02: No new packages — splitLink and httpSubscriptionLink are core @trpc/client v11 exports
+
+## Deviations from Plan
+
+### Auto-fixed Issues
+
+**1. [Rule 3 - Blocking] Duplicate useUtils() declaration in inbox.tsx**
+- **Found during:** Task 2 (inbox page)
+- **Issue:** Adding `const utils = trpc.useUtils()` for the subscription created a duplicate declaration (already existed lower in the function for mutations)
+- **Fix:** Moved the existing `useUtils()` call above the subscription hook and removed the duplicate
+- **Files modified:** packages/web/src/routes/inbox.tsx
+- **Verification:** `npm run build` succeeds
+- **Committed in:** 170ac55 (Task 2 commit)
+
+---
+
+**Total deviations:** 1 auto-fixed (1 blocking)
+**Impact on plan:** Trivial reorder of existing code. No scope creep.
+
+## Issues Encountered
+None
+
+## User Setup Required
+None - SSE works over standard HTTP through the existing Vite dev proxy.
+
+## Next Phase Readiness
+- Phase 20 (Real-Time Subscriptions) is now complete
+- All three UI pages receive live updates via SSE when domain events fire
+- Phase 21 (Frontend Polish) can proceed
+
+---
+*Phase: 20-real-time-subscriptions*
+*Completed: 2026-02-04*