Update documentation and CI to include world validation checks

- Added a note in AGENTS.md about using `mudtool validate -w ./world` for schema validation before committing.
- Updated TESTING.md to include a checklist item for the new validation command.
- Modified smoke-tests.yml to run the world validation command as part of the CI workflow.

.gitea/workflows/smoke-tests.yml

@@ -19,5 +19,8 @@ jobs:
       - name: Build
         run: cargo build
 
+      - name: Validate world data
+        run: ./target/debug/mudtool validate -w ./world
+
       - name: Run smoke tests
         run: ./run-tests.sh