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/tool_parallelism.rs

@@ -46,6 +46,7 @@ async fn run_turn(test: &TestCodex, prompt: &str) -> anyhow::Result<()> {
             model: session_model,
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })
@@ -361,6 +362,7 @@ async fn shell_tools_start_before_response_completed_when_stream_delayed() -> an
             model: session_model,
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })