add fast mode toggle (#13212)

- add a local Fast mode setting in codex-core (similar to how model id
is currently stored on disk locally)
- send `service_tier=priority` on requests when Fast is enabled
- add `/fast` in the TUI and persist it locally
- feature flag

codex-rs/tui/src/app_event.rs

@@ -25,6 +25,7 @@ use crate::history_cell::HistoryCell;
 use codex_core::features::Feature;
 use codex_protocol::config_types::CollaborationModeMask;
 use codex_protocol::config_types::Personality;
+use codex_protocol::config_types::ServiceTier;
 use codex_protocol::openai_models::ReasoningEffort;
 use codex_protocol::protocol::AskForApproval;
 use codex_protocol::protocol::SandboxPolicy;
@@ -197,6 +198,11 @@ pub(crate) enum AppEvent {
         personality: Personality,
     },
 
+    /// Persist the selected service tier to the appropriate config.
+    PersistServiceTierSelection {
+        service_tier: Option<ServiceTier>,
+    },
+
     /// Open the device picker for a realtime microphone or speaker.
     OpenRealtimeAudioDeviceSelection {
         kind: RealtimeAudioDeviceKind,