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/exec_policy.rs

@@ -55,6 +55,7 @@ async fn submit_user_turn(
             model: session_model,
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode,
             personality: None,
         })
@@ -134,6 +135,7 @@ async fn execpolicy_blocks_shell_invocation() -> Result<()> {
             model: session_model,
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })