“Scores of beds lie empty due to a series of decisions that have sharply reduced the number of prisoners sent to halfway houses. The Bureau of Prisons says it is curbing overspending of past years and streamlining operations, but putting inmates in halfway houses or on home confinement is much cheaper than imprisonment. Now there is evidence the Bureau of Prisons, under Sessions’ direction, is actively discouraging the use of transitional supervision even under existing rules and the Presidents’ wishes for reform.”