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/memories/phase1.rs

@@ -13,6 +13,7 @@ use crate::rollout::policy::should_persist_response_item_for_memories;
 use codex_api::ResponseEvent;
 use codex_otel::OtelManager;
 use codex_protocol::config_types::ReasoningSummary as ReasoningSummaryConfig;
+use codex_protocol::config_types::ServiceTier;
 use codex_protocol::models::BaseInstructions;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::ResponseItem;
@@ -36,6 +37,7 @@ pub(in crate::memories) struct RequestContext {
     pub(in crate::memories) otel_manager: OtelManager,
     pub(in crate::memories) reasoning_effort: Option<ReasoningEffortConfig>,
     pub(in crate::memories) reasoning_summary: ReasoningSummaryConfig,
+    pub(in crate::memories) service_tier: Option<ServiceTier>,
     pub(in crate::memories) turn_metadata_header: Option<String>,
 }
 
@@ -144,6 +146,7 @@ impl RequestContext {
             otel_manager: turn_context.otel_manager.clone(),
             reasoning_effort: Some(phase_one::REASONING_EFFORT),
             reasoning_summary: turn_context.reasoning_summary,
+            service_tier: turn_context.config.service_tier,
         }
     }
 }
@@ -322,6 +325,7 @@ mod job {
                 &stage_one_context.otel_manager,
                 stage_one_context.reasoning_effort,
                 stage_one_context.reasoning_summary,
+                stage_one_context.service_tier,
                 stage_one_context.turn_metadata_header.as_deref(),
             )
             .await?;