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/compact.rs

@@ -402,6 +402,7 @@ async fn drain_to_completed(
             &turn_context.otel_manager,
             turn_context.reasoning_effort,
             turn_context.reasoning_summary,
+            turn_context.config.service_tier,
             turn_metadata_header,
         )
         .await?;