Add TUI realtime conversation mode (#12687)

- Add a hidden `realtime_conversation` feature flag and `/realtime`
slash command for start/stop live voice sessions.
- Reuse transcription composer/footer UI for live metering, stream mic
audio, play assistant audio, render realtime user text events, and
force-close on feature disable.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/features.rs

@@ -144,6 +144,8 @@ pub enum Feature {
     Personality,
     /// Enable voice transcription in the TUI composer.
     VoiceTranscription,
+    /// Enable experimental realtime voice conversation mode in the TUI.
+    RealtimeConversation,
     /// Prevent idle system sleep while a turn is actively running.
     PreventIdleSleep,
     /// Use the Responses API WebSocket transport for OpenAI by default.
@@ -651,6 +653,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::RealtimeConversation,
+        key: "realtime_conversation",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::PreventIdleSleep,
         key: "prevent_idle_sleep",