Obsidian.

A free note-taking app where every note is a plain Markdown file sitting on your computer. Nothing is locked in the cloud, and you own all of it.

That detail is the whole point. AI reads markdown natively, so when you plug your Obsidian vault (your folder of notes) into Claude Code, Claude suddenly has a permanent memory of your entire business. Every doc, every process, every bit of context, all in a format it can read instantly.

It's brilliant for personal use, and it's where I kept my own Second brain for a long time.

The catch is scale. Once you're building a proper company brain with a team sharing it, a folder of local files starts to fall apart, and you'll want something built for that.