Capella Healthcare, a Franklin-based hospital chain, has agreed to sell its Parkway Medical Center in Decatur, Ala., to a not-for-profit health system. The 120-bed Parkway facility will become part of Huntsville Hospital Health System on Jan. 1. Starting next year, hospitals in Parkway's Alabama home county will be paid the lowest in the nation to treat Medicare patients. Amid such a tough reimbursement environment, Huntsville Hospital is the best partner for Parkway because of its network in the northern Alabama region that includes seven hospitals, said Capella Chief Executive Dan Slipkovich.