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/tui_app_server/src/main.rs

@@ -0,0 +1,41 @@
+use clap::Parser;
+use codex_arg0::Arg0DispatchPaths;
+use codex_arg0::arg0_dispatch_or_else;
+use codex_tui_app_server::Cli;
+use codex_tui_app_server::run_main;
+use codex_utils_cli::CliConfigOverrides;
+
+#[derive(Parser, Debug)]
+struct TopCli {
+    #[clap(flatten)]
+    config_overrides: CliConfigOverrides,
+
+    #[clap(flatten)]
+    inner: Cli,
+}
+
+fn main() -> anyhow::Result<()> {
+    arg0_dispatch_or_else(|arg0_paths: Arg0DispatchPaths| async move {
+        let top_cli = TopCli::parse();
+        let mut inner = top_cli.inner;
+        inner
+            .config_overrides
+            .raw_overrides
+            .splice(0..0, top_cli.config_overrides.raw_overrides);
+        let exit_info = run_main(
+            inner,
+            arg0_paths,
+            codex_core::config_loader::LoaderOverrides::default(),
+            None,
+        )
+        .await?;
+        let token_usage = exit_info.token_usage;
+        if !token_usage.is_zero() {
+            println!(
+                "{}",
+                codex_protocol::protocol::FinalOutput::from(token_usage),
+            );
+        }
+        Ok(())
+    })
+}