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/sqlite_state.rs

@@ -381,6 +381,7 @@ async fn mcp_call_marks_thread_memory_mode_polluted_when_configured() -> Result<
             model: test.session_configured.model.clone(),
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })