Move TUI on top of app server (parallel code) (#14717)

This PR replicates the `tui` code directory and creates a temporary
parallel `tui_app_server` directory. It also implements a new feature
flag `tui_app_server` to select between the two tui implementations.

Once the new app-server-based TUI is stabilized, we'll delete the old
`tui` directory and feature flag.

codex-rs/core/config.schema.json

@@ -488,6 +488,9 @@
             "tool_suggest": {
               "type": "boolean"
             },
+            "tui_app_server": {
+              "type": "boolean"
+            },
             "undo": {
               "type": "boolean"
             },
@@ -2037,6 +2040,9 @@
         "tool_suggest": {
           "type": "boolean"
         },
+        "tui_app_server": {
+          "type": "boolean"
+        },
         "undo": {
           "type": "boolean"
         },