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/lib.rs

@@ -421,6 +421,9 @@ impl Features {
                 "js_repl_tools_only" => {
                     continue;
                 }
+                "remote_control" => {
+                    continue;
+                }
                 "image_detail_original" => {
                     continue;
                 }
@@ -1117,7 +1120,7 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::RemoteControl,
         key: "remote_control",
-        stage: Stage::UnderDevelopment,
+        stage: Stage::Removed,
         default_enabled: false,
     },
     FeatureSpec {