docs: remove TDD focus, emphasize DRY and YAGNI

CLAUDE.md

@@ -14,6 +14,10 @@ If the import above did not load, read `AGENTS.md` at the repo root now — it i
 - Every project-owned PHP file starts with `declare(strict_types=1);` immediately after `<?php`.
 - Browser tests are CI-only (they run against a FrankenPHP container built by `docker build`, not `artisan serve`).
 
+## Design principles: DRY & YAGNI (not TDD)
+
+Write for the problem at hand, not an imagined future. **DRY**: extract and reuse only once logic is genuinely duplicated in more than one place. **YAGNI**: no speculative abstraction, configurability, or layers — add them only when a concrete requirement demands it. Prefer the simplest thing that solves the current requirement; avoid over-engineering and premature extraction. Tests are verification, not a design driver: write them to cover behavior already implemented, not as a front-loaded TDD ceremony.
+
 ## Environment note
 
 PHP and Composer are **not on PATH** in this environment, and `vendor/` and `node_modules/` are absent. Every `composer …` / `php artisan …` command in `AGENTS.md` and `README.md` assumes a PHP 8.4+ runtime with Composer 2 installed. Verify the toolchain before promising a command ran.