📝 Require concise prose in development/

Actionable sentence in CONTRIBUTING.md § Rules the tools don't enforce;
the why — humans skim, agents imitate the dominant style, so verbosity
compounds — in development/README.md § Decision blocks. Both sides in
one commit per the write-each-fact-once rule.

CONTRIBUTING.md

@@ -173,6 +173,9 @@ reaches for by default:
   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))
+- **Prose in `development/` is extremely concise.** When adding or changing a
+  decision, write fragments if needed — sacrifice grammar for concision. (why:
+  [development/README.md § Decision blocks](./development/README.md#decision-blocks))
 
 ## Branching model