fix: retain conversation retrieval settings for scheduled messages

Persist the conversation knowledge retrieval mode across restarts so timed messages use the saved setting. Clarify shared conversation authorization in the scheduled-task documentation and correct the renderer test query type.

Validation: 374 focused tests passed; typecheck and lint passed. The full suite had two failures that passed on focused reruns. Real remote validation remains blocked by the temporary harness SDK module loading error; no model requests were made.

Release note: 修复会话知识检索模式在重启后丢失的问题，定时消息继续使用会话保存的检索设置，并同步任务授权说明。

src/renderer/src/App.test.tsx

@@ -6403,7 +6403,7 @@ describe("App", () => {
     fireEvent.change(screen.getByLabelText("向 GoodBuddy 提问"), {
       target: { value: "Verify inherited model" },
     });
-    fireEvent.click(screen.getByRole("button", { name: "发送", exact: true }));
+    fireEvent.click(screen.getByRole("button", { name: "发送" }));
     await waitFor(() => {
       const saved = vi.mocked(api.conversations.saveLocal).mock.calls
         .flatMap(([conversations]) => conversations)