Not far into Kelly Reichardt’s “The Mastermind,” a very different kind of art heist film starring Josh O’Connor, the title’s irony becomes painfully clear. Because whatever one can say about J.B. Mooney, the mediocre art thief played by O’Connor with a brooding, hangdog sort of anti-energy, it’s obvious what one can’t. This onetime art student, now a family man and unemployed carpenter, is the furthest thing from a mastermind. He is neither clever, quick-thinking nor sensible — qualities one