Add AGENTS.md, point CLAUDE.md files at it

tests/CLAUDE.md

@@ -1,14 +1,3 @@
 # Writing tests
 
-Tests use [Brain\Monkey](https://brain-wp.github.io/BrainMonkey/) to stub WordPress functions without a full WP installation, and Mockery to mock `$wpdb` and other dependencies.
-
-All test classes extend `tests/Unit/TestCase.php`, which handles `Monkey\setUp()` / `Monkey\tearDown()` and stubs all WP translation/escape functions automatically.
-
-**Brain\Monkey API notes:**
-
-- `Functions\when('fn')->alias(fn() => ...)` — stub with a closure (NOT `returnUsing()`)
-- `Functions\when('fn')->justReturn($val)` — stub returning a fixed value
-- `Functions\expect('fn')->once()->with(...)` — assert call count and arguments
-- Use `Functions\when()` (not `Functions\expect()`) when you need argument-routing (e.g. `get_role` returning different values per argument) to avoid chaining ambiguity
-- Mockery matchers (e.g. `\Mockery::type()`) inside plain PHP arrays do not work with `with()` — use `\Mockery::on(fn($arr) => ...)` or `\Mockery::any()` instead
-- When mocking `$wpdb`, set `$mock->prefix = 'wp_'` explicitly — it is a public property, not a method
+See `../../AGENTS.md` (Tests section) — it is the single source of truth for working in this repo.