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/flowpilot/FlowPilotStepCard.tsx

@@ -1,5 +1,5 @@
 import { useState } from 'react'
-import { MessageSquare, Zap, CheckCircle2, SkipForward, ChevronDown, ChevronUp } from 'lucide-react'
+import { MessageSquare, Zap, CheckCircle2, SkipForward, ChevronDown, ChevronUp, GitFork } from 'lucide-react'
 import { cn } from '@/lib/utils'
 import type { AISessionStepResponse, StepResponseRequest } from '@/types/ai-session'
 import { isScriptGenerationAction, isScriptBuilderAction, getActionType } from '@/types/ai-session'
@@ -13,6 +13,8 @@ interface FlowPilotStepCardProps {
   isProcessing: boolean
   sessionId?: string
   onRespond: (response: StepResponseRequest) => void
+  onBranchSwitch?: (branchId: string) => void
+  activeBranchId?: string | null
 }
 
 const STEP_TYPE_ICONS = {
@@ -23,9 +25,10 @@ const STEP_TYPE_ICONS = {
   info_request: MessageSquare,
   script_generation: Zap,
   note: MessageSquare,
+  fork: GitFork,
 } as const
 
-export function FlowPilotStepCard({ step, isCurrentStep, isProcessing, sessionId, onRespond }: FlowPilotStepCardProps) {
+export function FlowPilotStepCard({ step, isCurrentStep, isProcessing, sessionId, onRespond, onBranchSwitch, activeBranchId }: FlowPilotStepCardProps) {
   const [isCollapsed, setIsCollapsed] = useState(!isCurrentStep)
 
   const content = step.content as Record<string, unknown>
@@ -94,6 +97,65 @@ export function FlowPilotStepCard({ step, isCurrentStep, isProcessing, sessionId
     )
   }
 
+  // Fork step — special rendering with branch options
+  if (contentType === 'fork') {
+    const forkReason = (content.fork_reason as string) || stepText
+    const forkBranches = (content.fork_branches as Array<{ branch_id: string; label: string }>) || []
+
+    return (
+      <div className="card-flat p-3 sm:p-4 lg:p-5 border-accent/30">
+        {/* Context message */}
+        {step.context_message && (
+          <div className="mb-3 rounded-lg bg-primary/5 px-3 py-2 border border-primary/10">
+            <MarkdownContent content={step.context_message} className="text-xs text-muted-foreground" />
+          </div>
+        )}
+
+        {/* Fork header */}
+        <div className="flex items-center gap-2 mb-3">
+          <span className="flex h-7 w-7 shrink-0 items-center justify-center rounded-lg bg-accent-dim">
+            <GitFork size={14} className="text-accent" />
+          </span>
+          <span className="text-[10px] font-semibold uppercase tracking-wider text-accent-text">
+            Diagnostic Fork
+          </span>
+        </div>
+
+        {/* Fork reason */}
+        <MarkdownContent content={forkReason} className="text-sm mb-4" />
+
+        {/* Branch options */}
+        {forkBranches.length > 0 && onBranchSwitch && (
+          <div className="flex flex-col gap-2">
+            {forkBranches.map((branch) => {
+              const isActive = branch.branch_id === activeBranchId
+              return (
+                <button
+                  key={branch.branch_id}
+                  onClick={() => onBranchSwitch(branch.branch_id)}
+                  className={cn(
+                    'w-full text-left rounded-[5px] border px-3 py-2.5 transition-colors',
+                    'hover:bg-elevated',
+                    isActive
+                      ? 'border-accent bg-accent-dim'
+                      : 'border-default bg-elevated/50'
+                  )}
+                >
+                  <p className={cn(
+                    'text-sm font-medium',
+                    isActive ? 'text-accent-text' : 'text-heading'
+                  )}>
+                    {branch.label}
+                  </p>
+                </button>
+              )
+            })}
+          </div>
+        )}
+      </div>
+    )
+  }
+
   // Current active step
   return (
     <div