enable/disable remote control at runtime, not via features (#22578)

## Why
reapplies https://github.com/openai/codex/pull/22386 which was
previously reverted

Also, introduce `remoteControl/enable` and `remoteControl/disable`
app-server APIs to toggle on/off remote control at runtime for a given
running app-server instance.

## What Changed

- Adds experimental v2 RPCs:
  - `remoteControl/enable`
  - `remoteControl/disable`
- Adds `RemoteControlRequestProcessor` and routes the new RPCs through
it instead of `ConfigRequestProcessor`.
- Adds named `RemoteControlHandle::enable`, `disable`, and `status`
methods.
- Makes `remoteControl/enable` return an error when sqlite state DB is
unavailable, while keeping enrollment/websocket failures as async status
updates.
- Adds `AppServerRuntimeOptions.remote_control_enabled` and hidden
`--remote-control` flags for `codex app-server` and `codex-app-server`.
- Updates managed daemon startup to use `codex app-server
--remote-control --listen unix://`.
- Marks `Feature::RemoteControl` as removed and ignores
`[features].remote_control`.
- Updates app-server README entries for the new remote-control methods.

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

@@ -30,6 +30,7 @@ use crate::request_processors::MarketplaceRequestProcessor;
 use crate::request_processors::McpRequestProcessor;
 use crate::request_processors::PluginRequestProcessor;
 use crate::request_processors::ProcessExecRequestProcessor;
+use crate::request_processors::RemoteControlRequestProcessor;
 use crate::request_processors::SearchRequestProcessor;
 use crate::request_processors::ThreadGoalRequestProcessor;
 use crate::request_processors::ThreadRequestProcessor;
@@ -173,6 +174,7 @@ pub(crate) struct MessageProcessor {
     marketplace_processor: MarketplaceRequestProcessor,
     mcp_processor: McpRequestProcessor,
     plugin_processor: PluginRequestProcessor,
+    remote_control_processor: RemoteControlRequestProcessor,
     search_processor: SearchRequestProcessor,
     thread_goal_processor: ThreadGoalRequestProcessor,
     thread_processor: ThreadRequestProcessor,
@@ -389,6 +391,7 @@ impl MessageProcessor {
             config_manager.clone(),
             workspace_settings_cache,
         );
+        let remote_control_processor = RemoteControlRequestProcessor::new(remote_control_handle);
         let search_processor = SearchRequestProcessor::new(outgoing.clone());
         let thread_goal_processor = ThreadGoalRequestProcessor::new(
             Arc::clone(&thread_manager),
@@ -446,7 +449,6 @@ impl MessageProcessor {
             auth_manager,
             thread_manager.clone(),
             analytics_events_client,
-            remote_control_handle,
         );
         let external_agent_config_processor = ExternalAgentConfigRequestProcessor::new(
             outgoing.clone(),
@@ -488,6 +490,7 @@ impl MessageProcessor {
             marketplace_processor,
             mcp_processor,
             plugin_processor,
+            remote_control_processor,
             search_processor,
             thread_goal_processor,
             thread_processor,
@@ -886,6 +889,14 @@ impl MessageProcessor {
                     .experimental_feature_enablement_set(request_id.clone(), params)
                     .await
             }
+            ClientRequest::RemoteControlEnable { .. } => self
+                .remote_control_processor
+                .enable()
+                .map(|response| Some(response.into())),
+            ClientRequest::RemoteControlDisable { .. } => self
+                .remote_control_processor
+                .disable()
+                .map(|response| Some(response.into())),
             ClientRequest::ConfigRequirementsRead { params: _, .. } => self
                 .config_processor
                 .config_requirements_read()