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/main/assistant/assistant-database.test.ts

@@ -148,6 +148,7 @@ describe('AssistantDatabase', () => {
     const { messages, ...header } = conversation
     database.saveLocalConversations([{ header: {
       ...header, workMode: 'execute',
+      knowledgeRetrievalMode: 'always',
       runtimeSelection: { provider: 'continue' }
     }, messages }])
     const existing = database.createSchedule({
@@ -159,7 +160,8 @@ describe('AssistantDatabase', () => {
     database.close()
     database.initialize('C:\\Workspace')
     expect(database.getConversation(conversation.id)).toMatchObject({
-      workMode: 'execute', runtimeSelection: { provider: 'continue' }
+      workMode: 'execute', runtimeSelection: { provider: 'continue' },
+      knowledgeRetrievalMode: 'always'
     })
     database.close()
   })