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/core/src/features.rs

@@ -145,6 +145,8 @@ pub enum Feature {
     CollaborationModes,
     /// Enable personality selection in the TUI.
     Personality,
+    /// Enable Fast mode selection in the TUI and request layer.
+    FastMode,
     /// Enable voice transcription in the TUI composer.
     VoiceTranscription,
     /// Enable experimental realtime voice conversation mode in the TUI.
@@ -660,6 +662,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::FastMode,
+        key: "fast_mode",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::VoiceTranscription,
         key: "voice_transcription",