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/types/ai-session.ts

@@ -221,9 +221,30 @@ export interface ChatMessageRequest {
   upload_ids?: string[]
 }
 
+export interface ForkMetadata {
+  fork_point_id: string
+  fork_reason: string
+  branches: Array<{ branch_id: string; label: string }>
+  active_branch_id: string
+}
+
+export interface ActionItem {
+  label: string
+  command?: string | null
+  description: string
+}
+
+export interface QuestionItem {
+  text: string
+  context?: string
+}
+
 export interface ChatMessageResponse {
   content: string
   suggested_flows: Array<{ tree_id: string; tree_name: string; tree_type: string; relevance_snippet: string }>
+  fork?: ForkMetadata | null
+  actions?: ActionItem[] | null
+  questions?: QuestionItem[] | null
 }
 
 export interface SimilarSession {