NEW YORK — Training to work in maximum-security prisons includes stern warnings that add up to one message: Don’t get too close to the inmates. Never treat the relationship as anything other than professional. Never reveal personal details an inmate could use to compromise you. And never forget you are dealing with hardened, often cunning, criminals. Investigators say prison tailor shop instructor Joyce Mitchell ignored those admonitions with frightening consequences, actually helping a pair of convicted killers make their power-tool breakout from upstate New York’s Clinton Correctional Facility.