### Preliminary /plugins TUI menu
- Adds a preliminary /plugins menu flow in both tui and tui_app_server.
- Fetches plugin list data asynchronously and shows loading/error/cached
states.
- Limits this first pass to the curated ChatGPT marketplace.
- Shows available plugins with installed/status metadata.
- Supports in-menu search over plugin display name, plugin id, plugin
name, and marketplace label.
- Opens a plugin detail view on selection, including summaries for
Skills, Apps, and MCP Servers, with back navigation.
### Testing
- Launch codex-cli with plugins enabled (`--enable plugins`).
- Run /plugins and verify:
- loading state appears first
- plugin list is shown
- search filters results
- selecting a plugin opens detail view, with a list of
skills/connectors/MCP servers for the plugin
- back action returns to the list.
- Verify disabled behavior by running /plugins without plugins enabled
(shows “Plugins are disabled” message).
- Launch with `--enable tui_app_server` (and plugins enabled) and repeat
the same /plugins flow; behavior should match.
This PR replicates the `tui` code directory and creates a temporary
parallel `tui_app_server` directory. It also implements a new feature
flag `tui_app_server` to select between the two tui implementations.
Once the new app-server-based TUI is stabilized, we'll delete the old
`tui` directory and feature flag.
