Most "second brains" are filing cabinets. You do the filing, and the brain does nothing.
Mine writes itself. I ask it something, it reads my Slack, my mail, my meeting transcripts, answers with an actual interface instead of four paragraphs of markdown, and files the part worth keeping as a linked note on my own disk. A month in, it knows my clients, my decisions and why I made them - so I never brief it twice.
Then the part I didn't expect: ask for the same thing twice and it builds you a tool for it. Your own panel, with its own agent inside. No code. It now does things no product I could have bought does - not because I designed them, but because they accumulated.
Still early and rough, but you can try it right now. Free, runs entirely on your own, and the notes are plain markdown so nothing is locked in.
https://github.com/emre-aktas/second-brain