[codex] Enable remote plugins by default (#30297)

## Summary

- enable the remote plugin feature by default
- promote the remote plugin feature from under development to stable
- preserve the existing `features.remote_plugin` override for explicitly
disabling it
- keep legacy disabled-path coverage explicit in TUI and app-server
tests

## Impact

Remote plugin functionality is enabled by default for configurations
that do not set the feature flag. The existing Codex backend
authentication gate still applies.

## Validation

- `just fmt`
- `just test -p codex-features`
- `just test -p codex-tui
plugins_popup_remote_section_fallback_states_snapshot`
- targeted `codex-app-server` plugin-list and skills-list tests
- `git diff --check`

The full TUI and app-server suites were also exercised locally. All
remote-plugin-related coverage passed; unrelated local
sandbox/test-binary failures remain outside this change.

codex-rs/core/src/plugins/discoverable_tests.rs

@@ -69,7 +69,6 @@ async fn list_tool_suggest_discoverable_plugins_includes_cached_remote_global_pl
         &codex_home.path().join(crate::config::CONFIG_TOML_FILE),
         r#"[features]
 plugins = true
-remote_plugin = true
 "#,
     );
 
@@ -306,7 +305,6 @@ remote_plugin = true
         &codex_home.path().join(crate::config::CONFIG_TOML_FILE),
         r#"[features]
 plugins = true
-remote_plugin = true
 
 [tool_suggest]
 disabled_tools = [