The F.B.I. had as many as eight informants inside the far-right Proud Boys in the months surrounding the storming of the Capitol on Jan. 6, 2021, recent court papers indicate, raising questions about how much federal investigators were able to learn from them about the violent mob attack both before and after it took place. The existence of the informants came to light over the past few days in a flurry of veiled court filings by defense lawyers for five members of the Proud Boys who are set to go on trial next month on seditious conspiracy charges connected to the Capitol attack.