Database

A spreadsheet is a fair mental picture: tables, rows, columns. What matters is that several things can read and write it at once without trampling each other, and it can answer "unfulfilled orders from last week" in a blink.

Your own tool will have one: the drafts it wrote, the notes you added, what it has already looked at.

Two things people get wrong. The database does not sit in your project folder next to the code, so saving the code does not save it. And it holds the part you cannot recreate by rebuilding, which is why it needs its own backup.