feat(client): add controllable session workspace - #21

Summary

• add a reusable useAgentSessions controller and let SessionList share it
• support controlled and uncontrolled active-session and session-list visibility in AgentChat
• keep the selected session visible while folded and forward turn-complete events
• align controls with existing labels, class names, and CSS variables, with host-owned persistence documented

Compatibility

• existing selection behavior remains uncontrolled by default
• the session list still starts open by default
• no browser storage or OpenOrange-specific styling is added to the shared package
• single-session embeddings do not load unused session data

Testing

• npm run check
• npm run typecheck
• npm run build
• npm test -w packages/agent-client (91 tests)