mark Feature::RemoteControl as removed (#22386)

## Why

`remote_control` can appear in `config.toml`, CLI feature overrides, and
the app-server config APIs. Before this PR, app-server startup treated
`config.features.enabled(Feature::RemoteControl)` as the signal to start
remote control ([base
code](https://github.com/openai/codex/blob/5e3ee5eddfa5333f2e0b011880abf0cbf92bd295/codex-rs/app-server/src/lib.rs#L678-L680)).
That meant a user with:

```toml
[features]
remote_control = true
```

would accidentally opt every app-server process into remote control.
Remote-control startup should instead be a per-process launch decision
made by CLI flags.

## What Changed

- Marks `Feature::RemoteControl` as `Stage::Removed`, keeping
`remote_control` as a known compatibility key while making it
config-inert.
- Adds a hidden `--remote-control` process flag to `codex app-server`
and standalone `codex-app-server`.
- Plumbs that flag through
`AppServerRuntimeOptions.remote_control_enabled` and makes app-server
startup use only that runtime option to decide whether to start remote
control.
- Removes the app-server config mutation hook that reloaded config and
toggled remote control at runtime.
- Updates managed daemon spawning to use `codex app-server
--remote-control --listen unix://` instead of `--enable remote_control`.

Config APIs can still list, read, write, and set `remote_control`; those
operations just no longer affect remote-control process enrollment.

codex-rs/app-server/src/message_processor.rs

@@ -42,7 +42,6 @@ use crate::skills_watcher::SkillsWatcher;
 use crate::thread_state::ConnectionCapabilities;
 use crate::thread_state::ThreadStateManager;
 use crate::transport::AppServerTransport;
-use crate::transport::RemoteControlHandle;
 use async_trait::async_trait;
 use codex_analytics::AnalyticsEventsClient;
 use codex_analytics::AppServerRpcTransport;
@@ -265,7 +264,6 @@ pub(crate) struct MessageProcessorArgs {
     pub(crate) auth_manager: Arc<AuthManager>,
     pub(crate) installation_id: String,
     pub(crate) rpc_transport: AppServerRpcTransport,
-    pub(crate) remote_control_handle: Option<RemoteControlHandle>,
     pub(crate) plugin_startup_tasks: crate::PluginStartupTasks,
 }
 
@@ -288,7 +286,6 @@ impl MessageProcessor {
             auth_manager,
             installation_id,
             rpc_transport,
-            remote_control_handle,
             plugin_startup_tasks,
         } = args;
         auth_manager.set_external_auth(Arc::new(ExternalAuthRefreshBridge {
@@ -446,7 +443,6 @@ impl MessageProcessor {
             auth_manager,
             thread_manager.clone(),
             analytics_events_client,
-            remote_control_handle,
         );
         let external_agent_config_processor = ExternalAgentConfigRequestProcessor::new(
             outgoing.clone(),