SAN FRANCISCO — Pacific Gas & Electric will plead guilty to 84 counts of involuntary manslaughter for a swath of death and destruction left behind after its fraying electrical grid ignited a 2018 wildfire that decimated three Northern California towns and drove the nation’s largest utility into bankruptcy. The plea agreement announced Monday resolves the charges facing PG&E in Butte County and marks the second time this decade that the company’s neglect has culminated in it being deemed a criminal.