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/suite/override_updates.rs

@@ -121,6 +121,7 @@ async fn override_turn_context_without_user_turn_does_not_record_permissions_upd
             model: None,
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })
@@ -161,6 +162,7 @@ async fn override_turn_context_without_user_turn_does_not_record_environment_upd
             model: None,
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })
@@ -198,6 +200,7 @@ async fn override_turn_context_without_user_turn_does_not_record_collaboration_u
             model: None,
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: Some(collaboration_mode),
             personality: None,
         })