feat: AI marker system prompt fixes, TaskLane activation, and FlowPilot updates

- Fix system prompt to ensure [QUESTIONS]/[ACTIONS] markers in AI responses
- Add format reminder injection to user messages for marker compliance
- Wire TaskLane activation in prefill and resume paths
- Add ActionCardGroup component for structured question/action rendering
- Update FlowPilot session and step card components
- Update ai-session schemas and types for marker data

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/session/BranchNode.tsx

@@ -78,7 +78,7 @@ export function BranchNode({ branch, depth, isActive, onClick }: BranchNodeProps
         type="button"
         onClick={() => onClick(branch.id)}
         className={cn(
-          'w-full text-left rounded-lg border p-2.5 transition-all duration-150',
+          'w-full text-left rounded-lg border p-2.5 transition-all duration-150 cursor-pointer',
           isActive
             ? cn('bg-card', config.borderClass)
             : 'bg-card/60 border-default hover:bg-card hover:border-hover',
@@ -102,8 +102,12 @@ export function BranchNode({ branch, depth, isActive, onClick }: BranchNodeProps
 
           {/* Expanded card positioned over the original */}
           <div
+            role="button"
+            tabIndex={0}
+            onClick={() => onClick(branch.id)}
+            onKeyDown={(e) => { if (e.key === 'Enter' || e.key === ' ') onClick(branch.id) }}
             className={cn(
-              'absolute z-50 inset-x-0 top-0',
+              'absolute z-50 inset-x-0 top-0 cursor-pointer',
               'bg-card border rounded-lg p-2.5',
               'shadow-[0_8px_32px_rgba(0,0,0,0.5)]',
               config.borderClass,