feat(06-04): render inline references and enforce 6-item limit

- keep reference previews inside textarea area with dedicated UI
- replace reference cap and toast copy from 10 to 6
- add e2e coverage for sixth-limit blocking behavior

frontend/src/components/workspace/input-box.tsx

@@ -89,7 +89,7 @@ import { Tooltip } from "./tooltip";
 import { useThread } from "./messages/context";
 import type { AppRouterInstance } from "next/dist/shared/lib/app-router-context.shared-runtime";
 
-const MAX_REFERENCES_PER_MESSAGE = 10;
+const MAX_REFERENCES_PER_MESSAGE = 6;
 
 type MentionCandidate = {
   key: string;
@@ -331,7 +331,7 @@ export function InputBox({
           return prev;
         }
         if (prev.length >= MAX_REFERENCES_PER_MESSAGE) {
-          toast.error("单条消息最多引用 10 个文件");
+          toast.error("单条消息最多引用 6 个文件");
           return prev;
         }
         return prev.concat({

frontend/tests/e2e/input-and-compose.spec.ts

@@ -155,4 +155,39 @@ test.describe("聊天工作台 / 输入区与发送", () => {
     // 该场景依赖特定后端/fixture 能制造 stale 引用；若环境无能力则显式跳过。
     testInfo.skip(true, "当前 E2E 环境无法稳定注入 stale 引用，使用 hooks 单测覆盖软失败逻辑。");
   });
+
+  test("DF-INPUT-009 引用上限为 6，第 7 个被阻止并提示", async (
+    { page },
+    testInfo,
+  ) => {
+    skipIfMissingThread(testInfo, THREAD_FOR_WELCOME, "FRONTEND_E2E_THREAD_ID");
+    await openChat(page, reuseThreadChatEntry(THREAD_FOR_WELCOME!));
+
+    const textarea = page.locator("textarea[name='message']");
+    await textarea.click();
+    await textarea.fill("请参考这些文件 ");
+
+    await textarea.type("@");
+    const panel = page.getByTestId("mention-candidate-panel").first();
+    await expect(panel).toBeVisible();
+    const initialItems = panel.locator("button");
+    const candidateCount = await initialItems.count();
+    testInfo.skip(candidateCount < 7, "当前线程候选文件不足 7 个，无法验证上限。");
+
+    for (let i = 0; i < 6; i += 1) {
+      await textarea.type("@");
+      const currentPanel = page.getByTestId("mention-candidate-panel").first();
+      await expect(currentPanel).toBeVisible();
+      await currentPanel.locator("button").nth(i).click();
+    }
+
+    await expect(page.getByLabel("移除引用")).toHaveCount(6);
+
+    await textarea.type("@");
+    await expect(panel).toBeVisible();
+    await panel.locator("button").nth(6).click();
+
+    await expect(page.getByLabel("移除引用")).toHaveCount(6);
+    await expect(page.getByText("单条消息最多引用 6 个文件")).toBeVisible();
+  });
 });