A rule shared between repos can no longer drift apart unnoticed

Each repository in the fleet keeps its working agreements in an AGENTS.md that is loaded into every assistant session in that repository. Several of those rules are deliberately the same everywhere, and until now the only thing keeping them the same was someone remembering to carry each edit to all seven repositories by hand.

That worked until it didn't. Nothing compared the copies, so a repository missed in a rollout, or edited a week later, looked exactly like one that was up to date — and every session in it quietly followed a different rule.

Shared rules are now marked as such and compared by CI across every repository that carries them. A copy that goes missing, loses its markers, or differs by so much as a word turns the check red and names the repository and the exact difference. Spans that are genuinely local — where a repository keeps its own documentation, which modules it ships — are declared as such centrally, so they stay free to differ while everything around them is held identical. The comparison also runs on a daily schedule, because a rule can drift on a day when nobody opens a pull request.