init: add source code from src.zip

src/utils/systemPromptType.ts

@@ -0,0 +1,14 @@
+/**
+ * Branded type for system prompt arrays.
+ *
+ * This module is intentionally dependency-free so it can be imported
+ * from anywhere without risking circular initialization issues.
+ */
+
+export type SystemPrompt = readonly string[] & {
+  readonly __brand: 'SystemPrompt'
+}
+
+export function asSystemPrompt(value: readonly string[]): SystemPrompt {
+  return value as SystemPrompt
+}