feat: persist task lane across submits and session reloads

Task lane questions/actions are now saved to a pending_task_lane JSONB
column on ai_sessions, restoring them on session switch or page reload.
Partial submit no longer force-clears the lane — the AI response
controls what stays. Also removes redundant "New Session" button from
the sidebar (dashboard already provides this).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/hooks/useFlowPilotSession.ts

@@ -92,6 +92,7 @@ export function useFlowPilotSession(): UseFlowPilotSession {
         ticket_data: null,
         steps: [firstStep],
         conversation_messages: [],
+        pending_task_lane: null,
         is_branching: false,
         active_branch_id: null,
       })