Edit: I also treat email like a to-do list, but it defers to my knowledge management system (linked in my parent comment) which tries to be complete like the OP

Edit: I also treat email like a to-do list, but it defers to my knowledge management system (linked in my parent comment) which tries to be complete like the OP suggests, but is AGPL, and has a similar but broader vision (not all features present yet, but hoping, and I use it every day). In it, every bit of knowledge is treated like the real-life "thing" that it is, and linked with everything else desired.