[codex] Ignore local curated plugins when remote catalog is active

[xl-openai]

Summary

• suppress configured openai-curated plugins when the remote plugin feature is enabled and auth uses the Codex backend
• preserve openai-api-curated and non-Codex-backend behavior while including remote catalog activation in the plugin load cache key
• add core plugin coverage and an app-server integration test for runtime feature enablement

Why

The Codex app enables remote plugins through process-local runtime feature enablement, which can happen after app-server startup tasks have already observed legacy local plugin state. The existing conflict logic only preferred a remote plugin when the same plugin was already installed remotely, so a configured legacy-only plugin could continue exposing skills and other capabilities from openai-curated.

Impact

When the remote catalog is active, legacy openai-curated plugins no longer contribute skills, MCP servers, apps, or hooks. Remote installed plugins continue to load normally, and openai-api-curated remains unaffected. This does not change remote fetch, bundle sync, or uninstall behavior.

Validation

• just test -p codex-core-plugins remote_global_catalog_ignores_local_curated_plugins remote_plugin_feature_keeps_local_curated_without_codex_backend
• just test -p codex-app-server runtime_remote_plugin_enablement_excludes_local_curated_plugin_skills
• just fmt
• git diff --check