* Triggers weak-side CU for first time since band set in 2005.: HONG KONG (Reuters) - Hong Kong’s central bank intervened in the market for the first time since 2015 after its currency hit the weaker end of its trading range, nudging up a key lending rate that could push borrowing costs higher. “I reiterate that the HKMA will buy Hong Kong dollars (HKD) and sell U.S. dollars at 7.85 level to ensure that the HKD exchange rate will not weaken beyond 7.8500,” Norman Chan, chief executive of the Hong Kong Monetary Authority (HKMA), said in a statement.