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/tasks/regular.rs

@@ -44,6 +44,7 @@ impl RegularTask {
                 &turn_context.otel_manager,
                 turn_context.reasoning_effort,
                 turn_context.reasoning_summary,
+                turn_context.config.service_tier,
                 turn_metadata_header.as_deref(),
             )
             .await?;