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/compact_remote.rs

@@ -1926,6 +1926,7 @@ async fn snapshot_request_shape_remote_pre_turn_compaction_including_incoming_us
                     model: None,
                     effort: None,
                     summary: None,
+                    service_tier: None,
                     collaboration_mode: None,
                     personality: None,
                 })
@@ -2035,6 +2036,7 @@ async fn snapshot_request_shape_remote_pre_turn_compaction_strips_incoming_model
             model: Some(next_model.to_string()),
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })