📚 Split backend testing memory and link from testing skill

Extract the backend Testing section from backend/core into a dedicated
backend/testing memory, following the pattern of common, frontend, and
exporter. Update the testing skill and root testing memory to point at
the new location, and add exporter/testing to the skill's required
reading list.

AI-assisted-by: deepseek-v4-flash

.opencode/skills/testing/SKILL.md

@@ -34,7 +34,8 @@ Before writing any test, read:
 2. Module-specific testing memory for the affected module:
    - `mem:common/testing` — CLJC unit tests
    - `mem:frontend/testing` — CLJS unit tests, Playwright E2E
-   - `mem:backend/core` — JVM clojure.test conventions
+   - `mem:backend/testing` — JVM clojure.test conventions
+   - `mem:exporter/testing` — exporter unit tests
 
 ## Key Rules