fix: replace magic numbers with named constants in polling logic

Address PR review feedback:
- Use WS_ACTIVITY_THRESHOLD constant instead of hardcoded 10000 in agent-info-panel.tsx
- Extract AGENT_OUTPUT_POLLING_INTERVAL constant for 5000ms value in use-features.ts

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/ui/src/components/views/board-view/components/kanban-card/agent-info-panel.tsx

@@ -117,7 +117,7 @@ export const AgentInfoPanel = memo(function AgentInfoPanel({
     }
     // If receiving WebSocket events, use longer polling interval as fallback
     if (isReceivingWsEvents) {
-      return 10000;
+      return WS_ACTIVITY_THRESHOLD;
     }
     // Default polling interval
     return 3000;