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/types/ai-session.ts

@@ -195,6 +195,7 @@ export interface AISessionDetail extends AISessionSummary {
   ticket_data: Record<string, unknown> | null
   steps: AISessionStepResponse[]
   conversation_messages: Array<{ role: string; content: string }>
+  pending_task_lane: { questions: QuestionItem[]; actions: ActionItem[] } | null
   is_branching: boolean
   active_branch_id: string | null
 }