AGENTS.md is the file coding agents read before they start work. Answer a few questions and this writes one for your project: an overview, the commands, your conventions, what must pass before a change is done, the decisions already made, and what an agent must ask about before changing.
Free, with no account. Everything happens in the page, nothing is uploaded, and anything you skip becomes a heading to fill in later.
More about the file: the AGENTS.md template and what to put in it.