feat: add real-time notification listener with WebSocket and polling fallback

[nooblemon-eth]

Summary

Add real-time notification system to the CLI that subscribes to the backend's existing notificationCreated GraphQL subscription.

Architecture:

• WebSocket subscription (src/api/subscriptions.ts): Connects to backend via graphql-ws, authenticates with bearer token, exponential backoff reconnection
• Polling fallback (src/chat/notifications/poller.ts): When WebSocket unavailable, polls getNotifications every 60s with deduplication
• Unified listener (src/chat/notifications/listener.ts): Tries WebSocket first, auto-falls back to polling after 5s disconnect
• Formatter (src/chat/notifications/formatter.ts): Renders notifications for CLI and MCP display
• MCP integration: lemonade mcp --watch starts the listener, surfaces notifications via MCP logging messages

Usage:

{
  "mcpServers": {
    "lemonade": {
      "command": "lemonade",
      "args": ["mcp", "--watch"]
    }
  }
}

No backend changes required — uses existing infrastructure:

• Backend already publishes all 48 notification types via Redis PubSub → GraphQL subscription
• Backend already has WebSocket support at /graphql endpoint

Test plan

• yarn build passes
• yarn test passes (1367 tests)
• Formatter tests (3): full fields, minimal fields, MCP JSON output
• Poller tests (4): interval polling, deduplication, stop cleanup, seen filtering
• Subscription module exports verified