[codex] Refactor sessions screen state

[roackb2]

Summary

Refactors useSessionsScreenState into focused session-screen hooks while preserving its public API.

• Moves shared state types into sessions-screen/sessionStateTypes.
• Extracts optimistic/live message helpers into useLiveSessionMessages.
• Extracts session detail loading, subscription, and live event handling into useSessionDetailSubscription.
• Extracts session creation/send/continue/cancel/settings/approval mutations into useSessionMutations.
• Extracts turn selection and trace-backed review loading into useSessionTurnReview.
• Replaces long live-event and trace-event if chains with extensible handler maps.

Impact

The existing app-facing hook shape remains unchanged, but the implementation is split by responsibility and is easier to extend without growing the facade or event dispatch logic.

Validation

• yarn vitest run src/__tests__/integration/web/control-plane-sessions-state.test.tsx src/__tests__/integration/web/control-plane-sessions-screen.test.tsx
• yarn typecheck
• yarn test
• yarn eslint
• yarn build