Make AGENTS.md a living document that tracks the project's actual shape

The guardrail document is copied once at scaffold time and never changes again, but almost everything in it is a statement about the project as it stood in that minute. It teaches the Bruno layout by pointing at the `example/` domain, and carries the full `auth` declaration table and endpoint-registration sample — so after `gonext add domain orders` (#15) it still sends an agent to `example/`, and if a project can ever be generated without auth, its whole security section describes something that isn't there.

The idea is for `AGENTS.md` to stop being a copied file and become a composed one: sections owned by the features actually present, with generators updating and appending rather than relying on the developer to remember. The obvious tension is that this pushes the file toward "generated" while developers will legitimately want to write their own content in it, so it probably needs an explicit boundary between a managed region and a free one — that boundary is the main open question.

Came out of an ideation session around #13, not a scoped request.

Start by tracing how scaffold generation creates AGENTS.md and how `gonext add domain orders` updates generated projects. Define the managed and user-editable boundaries, then verify that generated sections reflect enabled features such as domains and auth without overwriting custom content.

Written by the indexing model from the issue text.