init: add source code from src.zip

src/utils/userAgent.ts

@@ -0,0 +1,10 @@
+/**
+ * User-Agent string helpers.
+ *
+ * Kept dependency-free so SDK-bundled code (bridge, cli/transports) can
+ * import without pulling in auth.ts and its transitive dependency tree.
+ */
+
+export function getClaudeCodeUserAgent(): string {
+  return `claude-code/${MACRO.VERSION}`
+}