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/tests/common/mcp_process.rs

@@ -570,6 +570,18 @@ impl McpProcess {
             .await
     }
 
+    /// Send a `remoteControl/enable` JSON-RPC request.
+    pub async fn send_remote_control_enable_request(&mut self) -> anyhow::Result<i64> {
+        self.send_request("remoteControl/enable", /*params*/ None)
+            .await
+    }
+
+    /// Send a `remoteControl/disable` JSON-RPC request.
+    pub async fn send_remote_control_disable_request(&mut self) -> anyhow::Result<i64> {
+        self.send_request("remoteControl/disable", /*params*/ None)
+            .await
+    }
+
     /// Send an `app/list` JSON-RPC request.
     pub async fn send_apps_list_request(&mut self, params: AppsListParams) -> anyhow::Result<i64> {
         let params = Some(serde_json::to_value(params)?);