Handling a security advisory

Private vulnerability reporting is enabled on this repository and SECURITY.md routes every report to it. A report therefore arrives as a repository security advisory, never as an issue, and for most of its life it must not become one: a public issue would disclose the vulnerability before a fix exists.

Two steps in this flow are outward-facing, and both stay human-only: accepting an advisory (the reporter sees it) and publishing it (it becomes public, and there is no unpublish). Closing one as invalid, and every reply to a reporter, is outward-facing too. An agent never takes any of these steps: not once, not in bulk, not because a checklist said to. It investigates, prepares a recommendation, and a maintainer acts on it. Everything else here is mechanics.