Add story 53: WebSocket push notifications for panel auto-refresh

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.story_kit/stories/upcoming/53_websocket_push_notifications_for_ui.md

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+---
+name: WebSocket Push Notifications for Panel Auto-Refresh
+test_plan: pending
+---
+
+# Story 53: WebSocket Push Notifications for Panel Auto-Refresh
+
+## User Story
+
+As a user, I want the UI panels to update automatically when server-side state changes, so that I don't have to manually click Refresh to see current data.
+
+## Context
+
+The WebSocket at `/ws` already exists for chat streaming. Server-side deterministic commands (create_story, create_bug, close_bug, record_tests, ensure_acceptance, start_agent, etc.) mutate state that the panels display. Currently, panels only update on mount or manual refresh.
+
+By pushing lightweight notification events over the existing WebSocket when mutations occur, the frontend can auto-refresh the affected panel without polling.
+
+## Acceptance Criteria
+
+- [ ] The server broadcasts a notification event over the WebSocket when any of these mutations occur:
+  - Story created, moved, or archived
+  - Bug created or closed
+  - Tests recorded or acceptance checked
+  - Coverage collected
+  - Agent started, completed, or failed
+- [ ] Notification events are simple typed messages (e.g. `{"type": "notification", "topic": "stories"}`) — not full data payloads
+- [ ] The frontend listens for notification events and auto-refreshes the relevant panel:
+  - `stories` → UpcomingPanel, ReviewPanel
+  - `bugs` → (future BugPanel, no-op for now)
+  - `tests` / `acceptance` → GatePanel, TodoPanel
+  - `agents` → AgentPanel
+- [ ] Manual refresh buttons continue to work as before
+- [ ] Panels do not flicker or lose scroll position on auto-refresh
+
+## Out of Scope
+
+- File-system watching (inotify/fsevents) for changes made outside the server
+- Replacing the existing AgentPanel SSE with WebSocket (it works fine as-is)
+- Debouncing rapid successive notifications (can be a follow-up)