refactor: add typed content helpers for FlowPilot step card actions

Replace unsafe `as string` casts with type guard functions
(isScriptGenerationAction, isScriptBuilderAction, getActionType)
for compile-time safety on step content parsing.

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

frontend/src/types/ai-session.ts

@@ -41,6 +41,33 @@ export interface AISessionStepResponse {
   confidence_score: number
 }
 
+// ── Step content type guards ──
+
+export interface ScriptGenerationContent {
+  action_type: 'script_generation'
+  template_id?: string
+  pre_filled_params?: Record<string, string>
+  instructions?: string
+}
+
+export interface ScriptBuilderContent {
+  action_type: 'open_script_builder'
+  script_prompt?: string
+  script_language?: string
+}
+
+export function isScriptGenerationAction(content: Record<string, unknown>): content is ScriptGenerationContent {
+  return content.action_type === 'script_generation'
+}
+
+export function isScriptBuilderAction(content: Record<string, unknown>): content is ScriptBuilderContent {
+  return content.action_type === 'open_script_builder'
+}
+
+export function getActionType(content: Record<string, unknown>): string | undefined {
+  return typeof content.action_type === 'string' ? content.action_type : undefined
+}
+
 export interface StepResponseRequest {
   selected_option?: string | null
   free_text_input?: string | null