chore: load secrets from config/system/secrets.php

additional.php is maintained in Git now (taken over from DDEV's
generated file): DDEV settings apply inside DDEV only, and
secrets.php (gitignored, template secrets.example.php) is merged in
everywhere, ignoring empty values. The web container reaches the local
MyE&P project as myep-next.ddev.site, as in EP/website.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

.ddev/docker-compose.communicate.yaml

@@ -0,0 +1,5 @@
+# Reach the local MyE&P DDEV project from this web container (as in EP/website)
+services:
+  web:
+    external_links:
+      - "ddev-router:myep-next.ddev.site"