add top-level remote-control command (#21424)

## Summary

`codex --enable remote_control app-server --listen off` is the current
way to start a headless, remote-controllable app-server, but it is hard
to remember and exposes implementation details.

This adds `codex remote-control` as a friendly top-level wrapper for
that flow. The command starts a foreground app-server with local
transports disabled and enables `remote_control` only for that
invocation.

## Changes

- Add a visible `codex remote-control` CLI subcommand.
- Launch app-server with `AppServerTransport::Off`.
- Append `features.remote_control=true` after root feature toggles so
the explicit command wins over `--disable remote_control`.
- Reject root `--remote` / `--remote-auth-token-env`, matching other
non-TUI subcommands.
- Add tests for parsing, launch defaults, override ordering, and remote
flag rejection.

## Verification

- `cargo test -p codex-cli`
- `just fix -p codex-cli`

codex-rs/cli/src/main.rs

@@ -126,6 +126,9 @@ enum Subcommand {
     /// [experimental] Run the app server or related tooling.
     AppServer(AppServerCommand),
 
+    /// [experimental] Start a headless app-server with remote control enabled.
+    RemoteControl,
+
     /// Launch the Codex desktop app (opens the app installer if missing).
     #[cfg(any(target_os = "macos", target_os = "windows"))]
     App(app_cmd::AppCommand),
@@ -725,6 +728,14 @@ struct FeatureSetArgs {
     feature: String,
 }
 
+const REMOTE_CONTROL_FEATURE_OVERRIDE: &str = "features.remote_control=true";
+
+fn enable_remote_control_for_invocation(config_overrides: &mut CliConfigOverrides) {
+    config_overrides
+        .raw_overrides
+        .push(REMOTE_CONTROL_FEATURE_OVERRIDE.to_string());
+}
+
 fn stage_str(stage: Stage) -> &'static str {
     match stage {
         Stage::UnderDevelopment => "under development",
@@ -896,6 +907,24 @@ async fn cli_main(arg0_paths: Arg0DispatchPaths) -> anyhow::Result<()> {
                 }
             }
         }
+        Some(Subcommand::RemoteControl) => {
+            reject_remote_mode_for_subcommand(
+                root_remote.as_deref(),
+                root_remote_auth_token_env.as_deref(),
+                "remote-control",
+            )?;
+            enable_remote_control_for_invocation(&mut root_config_overrides);
+            codex_app_server::run_main_with_transport(
+                arg0_paths.clone(),
+                root_config_overrides,
+                codex_config::LoaderOverrides::default(),
+                /*default_analytics_enabled*/ false,
+                codex_app_server::AppServerTransport::Off,
+                codex_protocol::protocol::SessionSource::Cli,
+                codex_app_server::AppServerWebsocketAuthSettings::default(),
+            )
+            .await?;
+        }
         #[cfg(any(target_os = "macos", target_os = "windows"))]
         Some(Subcommand::App(app_cli)) => {
             reject_remote_mode_for_subcommand(
@@ -2276,6 +2305,45 @@ mod tests {
         assert!(app_server.analytics_default_enabled);
     }
 
+    #[test]
+    fn remote_control_override_is_appended_after_root_toggles() {
+        let mut config_overrides = CliConfigOverrides::default();
+        config_overrides
+            .raw_overrides
+            .push("features.remote_control=false".to_string());
+
+        enable_remote_control_for_invocation(&mut config_overrides);
+
+        assert_eq!(
+            config_overrides.raw_overrides,
+            vec![
+                "features.remote_control=false".to_string(),
+                REMOTE_CONTROL_FEATURE_OVERRIDE.to_string(),
+            ]
+        );
+    }
+
+    #[test]
+    fn reject_remote_flag_for_remote_control() {
+        let cli = MultitoolCli::try_parse_from([
+            "codex",
+            "--remote",
+            "ws://127.0.0.1:1234",
+            "remote-control",
+        ])
+        .expect("parse");
+        assert_matches!(cli.subcommand, Some(Subcommand::RemoteControl));
+
+        let err = reject_remote_mode_for_subcommand(
+            cli.remote.remote.as_deref(),
+            cli.remote.remote_auth_token_env.as_deref(),
+            "remote-control",
+        )
+        .expect_err("remote-control should reject root --remote");
+
+        assert!(err.to_string().contains("remote-control"));
+    }
+
     #[test]
     fn remote_flag_parses_for_interactive_root() {
         let cli = MultitoolCli::try_parse_from(["codex", "--remote", "ws://127.0.0.1:4500"])