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/src/config/profile.rs

@@ -8,6 +8,7 @@ use crate::config::types::WindowsToml;
 use crate::protocol::AskForApproval;
 use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::config_types::SandboxMode;
+use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::Verbosity;
 use codex_protocol::config_types::WebSearchMode;
 use codex_protocol::openai_models::ReasoningEffort;
@@ -18,6 +19,7 @@ use codex_protocol::openai_models::ReasoningEffort;
 #[schemars(deny_unknown_fields)]
 pub struct ConfigProfile {
     pub model: Option<String>,
+    pub service_tier: Option<ServiceTier>,
     /// The key in the `model_providers` map identifying the
     /// [`ModelProviderInfo`] to use.
     pub model_provider: Option<String>,