Jessica Fain's best product ideas kept dying, and she couldn't figure out why. So at eight and a half months pregnant, she pitched @SlackHQ's CPO @aunder on becoming her Chief of Staff. She wanted to see how executive decisions actually get made from the inside. What she learned changed everything she knew about influencing execs. People don't realize that an executive's calendar is like a strobe light going off. Budget meeting, a people problem, a legal issue—then your product review.…