## Summary
Remote plugin-service returns plugin availability separately from a
user's installed/enabled state. This adds `PluginAvailabilityStatus` to
the app-server protocol, propagates remote catalog `status` into
`PluginSummary`, and rejects install attempts for remote plugins marked
`DISABLED_BY_ADMIN` before downloading or caching the bundle.
This is the `openai/codex` half of the change. The companion
`openai/openai` webview PR is
https://github.com/openai/openai/pull/873269.
## Validation
- `cargo run -p codex-app-server-protocol --bin write_schema_fixtures`
- `cargo test -p codex-app-server --test all
plugin_list_marks_remote_plugin_disabled_by_admin`
- `cargo test -p codex-app-server --test all
plugin_list_includes_remote_marketplaces_when_remote_plugin_enabled`
- `cargo test -p codex-app-server --test all
plugin_install_rejects_remote_plugin_disabled_by_admin_before_download`
- `cargo test -p codex-app-server-protocol schema_fixtures`
1. Adds v2 plugin/share/save, plugin/share/list, and plugin/share/delete
RPCs.
2. Implements save by archiving a local plugin root, enforcing a size
limit, uploading through the workspace upload flow, and supporting
updates via remotePluginId.
3. Lists created workspace plugins
4. Deletes a previously uploaded/shared plugin.
