Add a #notes command, which opens up a blank text editor window where you can type anything you want. The notes are saved along with the game and can be referenced anytime from within it.

To prevent this being a hugely complicated undertaking (basically implementing a text editor in the game, which won’t mesh particularly well with players used to other text editors), one suggestion is to use the shell’s default editor, but then this becomes akin to just using out-of-game notes anyway.

Another suggestion is to implement it using a system similar to autopickup exceptions: the player can add, edit, list, and delete one-line notes, which doesn’t require implementing any text editing capability the game doesn’t already have.