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/config.schema.json

@@ -367,6 +367,9 @@
             "prevent_idle_sleep": {
               "type": "boolean"
             },
+            "realtime_conversation": {
+              "type": "boolean"
+            },
             "remote_models": {
               "type": "boolean"
             },
@@ -1657,6 +1660,9 @@
         "prevent_idle_sleep": {
           "type": "boolean"
         },
+        "realtime_conversation": {
+          "type": "boolean"
+        },
         "remote_models": {
           "type": "boolean"
         },