The Minnesota Nurses Association and the 14 Twin Cities Hospitals, following months of heated negotiations over a new contract, report that they have reached a tentative agreement.  The surprise agreement comes after the parties walked away from the table at 1 a.m. Wednesday, only to quietly return at 11:00 p.m.  The nurses union dropped its main demand in the negotiations -- nurse-patient ratios -- and agreed to the hospital's wage proposal: no wage increase the first year, and 1 and 2 percent increases in the next two years.  The hospital dropped its proposals to change pension and insurance benefits.  The tentative agreement must be ratified by union members.   The strike by over 12,000 Minnesota nurses was set to begin July 6.   Instead of striking, the nurses will vote on the contract July 6.