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-protocol/src/protocol/common.rs

@@ -799,6 +799,18 @@ client_request_definitions! {
         serialization: global("config"),
         response: v2::ExperimentalFeatureEnablementSetResponse,
     },
+    #[experimental("remoteControl/enable")]
+    RemoteControlEnable => "remoteControl/enable" {
+        params: #[ts(type = "undefined")] #[serde(skip_serializing_if = "Option::is_none")] Option<()>,
+        serialization: global("remote-control"),
+        response: v2::RemoteControlEnableResponse,
+    },
+    #[experimental("remoteControl/disable")]
+    RemoteControlDisable => "remoteControl/disable" {
+        params: #[ts(type = "undefined")] #[serde(skip_serializing_if = "Option::is_none")] Option<()>,
+        serialization: global("remote-control"),
+        response: v2::RemoteControlDisableResponse,
+    },
     #[experimental("collaborationMode/list")]
     /// Lists collaboration mode presets.
     CollaborationModeList => "collaborationMode/list" {

codex-rs/app-server-protocol/src/protocol/v2/remote_control.rs

@@ -13,6 +13,24 @@ pub struct RemoteControlStatusChangedNotification {
     pub environment_id: Option<String>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct RemoteControlEnableResponse {
+    pub status: RemoteControlConnectionStatus,
+    pub installation_id: String,
+    pub environment_id: Option<String>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct RemoteControlDisableResponse {
+    pub status: RemoteControlConnectionStatus,
+    pub installation_id: String,
+    pub environment_id: Option<String>,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(rename_all = "camelCase", export_to = "v2/")]
@@ -22,3 +40,33 @@ pub enum RemoteControlConnectionStatus {
     Connected,
     Errored,
 }
+
+impl From<RemoteControlStatusChangedNotification> for RemoteControlEnableResponse {
+    fn from(notification: RemoteControlStatusChangedNotification) -> Self {
+        let RemoteControlStatusChangedNotification {
+            status,
+            installation_id,
+            environment_id,
+        } = notification;
+        Self {
+            status,
+            installation_id,
+            environment_id,
+        }
+    }
+}
+
+impl From<RemoteControlStatusChangedNotification> for RemoteControlDisableResponse {
+    fn from(notification: RemoteControlStatusChangedNotification) -> Self {
+        let RemoteControlStatusChangedNotification {
+            status,
+            installation_id,
+            environment_id,
+        } = notification;
+        Self {
+            status,
+            installation_id,
+            environment_id,
+        }
+    }
+}