Aligned feature stage names with public feature maturity stages (#9929)

We've recently standardized a [feature maturity
model](https://developers.openai.com/codex/feature-maturity) that we're
using in our docs and support forums to communicate expectations to
users. This PR updates the internal stage names and descriptions to
match.

This change involves a simple internal rename and updates to a few
user-visible strings. No functional change.

codex-rs/cli/src/main.rs

@@ -453,8 +453,8 @@ enum FeaturesSubcommand {
 fn stage_str(stage: codex_core::features::Stage) -> &'static str {
     use codex_core::features::Stage;
     match stage {
-        Stage::Beta => "experimental",
-        Stage::Experimental { .. } => "beta",
+        Stage::UnderDevelopment => "under development",
+        Stage::Experimental { .. } => "experimental",
         Stage::Stable => "stable",
         Stage::Deprecated => "deprecated",
         Stage::Removed => "removed",