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/features/src/tests.rs

@@ -293,9 +293,24 @@ fn auth_elicitation_is_under_development() {
 }
 
 #[test]
-fn remote_control_is_under_development() {
-    assert_eq!(Feature::RemoteControl.stage(), Stage::UnderDevelopment);
+fn remote_control_is_removed_and_disabled_by_default() {
+    assert_eq!(Feature::RemoteControl.stage(), Stage::Removed);
     assert_eq!(Feature::RemoteControl.default_enabled(), false);
+    assert_eq!(
+        feature_for_key("remote_control"),
+        Some(Feature::RemoteControl)
+    );
+}
+
+#[test]
+fn remote_control_config_is_ignored() {
+    let mut entries = BTreeMap::new();
+    entries.insert("remote_control".to_string(), true);
+
+    let mut features = Features::with_defaults();
+    features.apply_map(&entries);
+
+    assert_eq!(features.enabled(Feature::RemoteControl), false);
 }
 
 #[test]