Remove the legacy TUI split (#15922)

This is the part 1 of 2 PRs that will delete the `tui` /
`tui_app_server` split. This part simply deletes the existing `tui`
directory and marks the `tui_app_server` feature flag as removed. I left
the `tui_app_server` feature flag in place for now so its presence
doesn't result in an error. It is simply ignored.

Part 2 will rename the `tui_app_server` directory `tui`. I did this as
two parts to reduce visible code churn.

codex-rs/features/src/lib.rs

@@ -176,7 +176,7 @@ pub enum Feature {
     VoiceTranscription,
     /// Enable experimental realtime voice conversation mode in the TUI.
     RealtimeConversation,
-    /// Route interactive startup to the app-server-backed TUI implementation.
+    /// Removed compatibility flag. The TUI now always uses the app-server implementation.
     TuiAppServer,
     /// Prevent idle system sleep while a turn is actively running.
     PreventIdleSleep,
@@ -371,10 +371,16 @@ impl Features {
                         Feature::WebSearchCached,
                     );
                 }
+                "tui_app_server" => {
+                    continue;
+                }
                 _ => {}
             }
             match feature_for_key(k) {
                 Some(feat) => {
+                    if matches!(feat, Feature::TuiAppServer) {
+                        continue;
+                    }
                     if k != feat.key() {
                         self.record_legacy_usage(k.as_str(), feat);
                     }
@@ -822,7 +828,7 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::TuiAppServer,
         key: "tui_app_server",
-        stage: Stage::Stable,
+        stage: Stage::Removed,
         default_enabled: true,
     },
     FeatureSpec {