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/tests/responses_headers.rs

@@ -118,6 +118,7 @@ async fn responses_stream_includes_subagent_header_on_review() {
             effort,
             summary.unwrap_or(model_info.default_reasoning_summary),
             None,
+            None,
         )
         .await
         .expect("stream failed");
@@ -230,6 +231,7 @@ async fn responses_stream_includes_subagent_header_on_other() {
             effort,
             summary.unwrap_or(model_info.default_reasoning_summary),
             None,
+            None,
         )
         .await
         .expect("stream failed");
@@ -341,6 +343,7 @@ async fn responses_respects_model_info_overrides_from_config() {
             effort,
             summary.unwrap_or(model_info.default_reasoning_summary),
             None,
+            None,
         )
         .await
         .expect("stream failed");