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/exec/src/lib.rs

@@ -567,6 +567,7 @@ pub async fn run_main(cli: Cli, arg0_paths: Arg0DispatchPaths) -> anyhow::Result
                     model: default_model,
                     effort: default_effort,
                     summary: None,
+                    service_tier: None,
                     final_output_json_schema: output_schema,
                     collaboration_mode: None,
                     personality: None,