The 31-story Peachtree Summit Federal Building in downtown Atlanta was evacuated yesterday after mailroom workers flagged a suspicious package that turned out to be a decorative egg. Federal Protection Service authorities ordered the evacuation just before noon Tuesday after a routine X-ray spotted what appeared at first to be a grenade inside the package. The suspicious item later turned out to be a Fabergé-like egg. About 1,900 federal employees from the IRS, Social Security Administration and other agencies work at the building, which also contains a daycare center. Employees were allowed to return to the building about an hour and…