refactor(core): extract shared usage, auth, and display helpers

src/shared/chat-message-content.ts

@@ -0,0 +1,15 @@
+export function extractFirstTextBlock(message: unknown): string | undefined {
+  if (!message || typeof message !== "object") {
+    return undefined;
+  }
+  const content = (message as { content?: unknown }).content;
+  if (!Array.isArray(content) || content.length === 0) {
+    return undefined;
+  }
+  const first = content[0];
+  if (!first || typeof first !== "object") {
+    return undefined;
+  }
+  const text = (first as { text?: unknown }).text;
+  return typeof text === "string" ? text : undefined;
+}