It’s AGPL. Do you mean “taking notes at work” or “fork the source and incorporate it into my non-AGPL licensed software”? The former of these is fine.

You can use the tool at work. You just can't incorporate the tool's code into your work's product.

Per the license you can incorporate the tool's code in to your work product, too, but only if you release your work product AGPL; your company (probably reasonably) won't let you, and might also (unreasonably) prohibit use.