📝 Make the rule/rationale split explicit

AGENTS.md and CONTRIBUTING.md now state that a decision, its rejected
alternatives and its known issues belong in development/<category>.md, that the
actionable rule stays in CONTRIBUTING.md, and that both change in the same
commit. development/README.md names library.md in the category list and spells
out that the sync runs in both directions.

CONTRIBUTING.md

@@ -125,6 +125,11 @@ reaches for by default:
 - **There is no local `npm run publish`, and `publish:publint` / `publish:attw`
   don't go in `check`.** (why:
   [development/publishing.md](./development/publishing.md#ci-only-publishing))
+- **A decision or its rationale belongs in `development/`, not here.** This file
+  holds the actionable rule; `development/<category>.md` holds why, the rejected
+  alternatives and the known issues. When you change a rule, update its category
+  file in the same commit and cross-link the two. (why:
+  [development/README.md](./development/README.md))
 
 ## Branching model