Register the transcript v2 feature flag (#40554)

## What changed

- Add the under-development `transcript_v2` feature for the interactive
  transcript composer and turn-selection UI.
- Keep it disabled by default and expose it through the configuration schema
  and `codex features enable transcript_v2`.

## Testing

- Verify explicit configuration overrides and CLI persistence for the new
  feature.

GitOrigin-RevId: 138ab7851621c066e8121cc3eb6cd4e7299f9c31

codex-rs/core/config.schema.json

@@ -931,6 +931,9 @@
             "tool_suggest": {
               "type": "boolean"
             },
+            "transcript_v2": {
+              "type": "boolean"
+            },
             "tui_app_server": {
               "type": "boolean"
             },
@@ -5934,6 +5937,9 @@
         "tool_suggest": {
           "type": "boolean"
         },
+        "transcript_v2": {
+          "type": "boolean"
+        },
         "tui_app_server": {
           "type": "boolean"
         },