refactor(agents): share text block extraction helper

src/agents/content-blocks.ts

@@ -0,0 +1,16 @@
+export function collectTextContentBlocks(content: unknown): string[] {
+  if (!Array.isArray(content)) {
+    return [];
+  }
+  const parts: string[] = [];
+  for (const block of content) {
+    if (!block || typeof block !== "object") {
+      continue;
+    }
+    const rec = block as { type?: unknown; text?: unknown };
+    if (rec.type === "text" && typeof rec.text === "string") {
+      parts.push(rec.text);
+    }
+  }
+  return parts;
+}