fix(ai): Ask P0 禁 CoT、可澄清、禁 UPDATE_TIME 终答

prompt 禁止推理外露与编造列；信息不足允许一句澄清。
新增 SanitizeAssistantContent 剥回复中的推理草稿；业务「最近 N 天」不得默认用 information_schema.UPDATE_TIME。

server/internal/ai/ai_test.go

@@ -118,6 +118,12 @@ func TestAskChatsWithoutClaimingDatabaseExecution(t *testing.T) {
 	if !strings.Contains(generate[0].Content, "生成一条 mysql 方言的 SQL") {
 		t.Fatalf("SQL generation lost its dedicated instruction: %s", generate[0].Content)
 	}
+	if !strings.Contains(generate[0].Content, "澄清") || strings.Contains(generate[0].Content, "只输出可执行 SQL") {
+		t.Fatalf("generate must allow clarification and not force executable-only: %s", generate[0].Content)
+	}
+	if !strings.Contains(generate[0].Content, "禁止编造") || !strings.Contains(generate[0].Content, "UPDATE_TIME") {
+		t.Fatalf("system rules missing invent/UPDATE_TIME guards: %s", generate[0].Content)
+	}
 	offline := chatStub(request)
 	if offline.SQL != "" || !strings.Contains(offline.Content, "离线体验无法回答") {
 		t.Fatalf("offline Ask must not pretend to generate or query: %+v", offline)