[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/features/src/lib.rs

@@ -190,7 +190,7 @@ pub enum Feature {
     ///
     /// Requirements-only gate: this should be set from requirements, not user config.
     ComputerUse,
-    /// Temporary internal-only flag for PS-backed remote plugin catalog development.
+    /// Enable the PS-backed remote plugin catalog.
     RemotePlugin,
     /// Enable remote plugin sharing flows.
     PluginSharing,
@@ -1145,8 +1145,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::RemotePlugin,
         key: "remote_plugin",
-        stage: Stage::UnderDevelopment,
-        default_enabled: false,
+        stage: Stage::Stable,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::PluginSharing,