Replace useState-based useHub hook and separate useActiveAgent store
with a single useHubStore (Zustand). This fixes the bug where
HubSidebar and Chat held independent state copies, causing stale data
and duplicate 30s polling. Agent create/delete logic now lives in the
store with automatic activeAgentId management.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>