📝 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.

development/README.md

@@ -76,10 +76,11 @@ of downloading per job.
 
 ## Adding to these docs
 
-1. Pick the category file for the area you are changing: `workflow`, `tooling`,
-   `testing`, `ci`, or `publishing`.
+1. Pick the category file for the area you are changing: `library`, `workflow`,
+   `tooling`, `testing`, `ci`, or `publishing`.
 2. Add or update a decision block. Keep the existing text unless the decision
    actually changed.
 3. If the change alters an actionable rule, update
    [CONTRIBUTING.md](../CONTRIBUTING.md) in the same commit and cross-link the
-   two.
+   two. The reverse also holds: never change a rule in CONTRIBUTING.md without
+   updating its rationale here.