The code is half of what you receive. The other half is how to change it.
A skill is a written procedure an AI assistant follows to do one job the way this codebase does it.
Scaffold a new command slice. Add an account type by extending an existing one. Write a component test the way the suite expects. Author a help article. Sync settings. There are thirty of them today, and they are how we build. They ship with the source, so your team extends the engine the way we do, from day one.
The documentation is kept current by the same process that ships the code.
A decision record is written before a change and marked shipped with what actually happened. A help article ships in the same commit as the screen it explains. The domain model and the capability atlas are generated from the code. Most vendors hand over a wiki that was true at go-live.
What this means for you: vendor risk becomes a choice you make, not a risk you carry. A two-person vendor is not a bus-factor problem when you hold the code, the tools and the history.