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/protocol/src/protocol.rs

@@ -18,6 +18,7 @@ use crate::config_types::CollaborationMode;
 use crate::config_types::ModeKind;
 use crate::config_types::Personality;
 use crate::config_types::ReasoningSummary as ReasoningSummaryConfig;
+use crate::config_types::ServiceTier;
 use crate::config_types::WindowsSandboxLevel;
 use crate::custom_prompts::CustomPrompt;
 use crate::dynamic_tools::DynamicToolCallOutputContentItem;
@@ -222,6 +223,15 @@ pub enum Op {
         /// fall back to the selected model's default on new sessions).
         #[serde(default, skip_serializing_if = "Option::is_none")]
         summary: Option<ReasoningSummaryConfig>,
+
+        /// Optional service tier override for this turn.
+        ///
+        /// Use `Some(Some(_))` to set a specific tier for this turn, `Some(None)` to
+        /// explicitly clear the tier for this turn, or `None` to keep the existing
+        /// session preference.
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        service_tier: Option<Option<ServiceTier>>,
+
         // The JSON schema to use for the final assistant message
         final_output_json_schema: Option<Value>,
 
@@ -274,6 +284,13 @@ pub enum Op {
         #[serde(skip_serializing_if = "Option::is_none")]
         summary: Option<ReasoningSummaryConfig>,
 
+        /// Updated service tier preference for future turns.
+        ///
+        /// Use `Some(Some(_))` to set a specific tier, `Some(None)` to clear the
+        /// preference, or `None` to leave the existing value unchanged.
+        #[serde(skip_serializing_if = "Option::is_none")]
+        service_tier: Option<Option<ServiceTier>>,
+
         /// EXPERIMENTAL - set a pre-set collaboration mode.
         /// Takes precedence over model, effort, and developer instructions if set.
         #[serde(skip_serializing_if = "Option::is_none")]