fix(ci): revert accidental third_party mount, use compose override for OIDC

The third_party:/opt/apps volume was accidentally uncommented in
docker-compose.yml. Without submodules checked out, this empty mount
breaks the Notes app installation hook in CI.

Fix: keep the mount commented in docker-compose.yml and add a separate
docker-compose.oidc.yml override that's only used for OIDC-requiring
profiles (oauth, login-flow) in CI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.oidc.yml

@@ -0,0 +1,7 @@
+# Override to mount the OIDC app into Nextcloud.
+# Use with: docker compose -f docker-compose.yml -f docker-compose.oidc.yml --profile <profile> up
+# Required for OAuth, Keycloak, and Login Flow profiles.
+services:
+  app:
+    volumes:
+      - ./third_party:/opt/apps:ro