Documentation that stays true to the code
Requirements, architecture and decisions live beside the code, and every change that alters behaviour, configuration or the build carries its documentation in the same reviewed change; review sends back a change that doesn't. Every link is checked on every pull request, and the checks fail on a requirement that is cited but defined nowhere, so each one keeps a single definition you can trace to the code that meets it. A new engineer, or a new AI tool, starts from the documents, not from guesswork.