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/permissions_messages.rs

@@ -120,6 +120,7 @@ async fn permissions_message_added_on_override_change() -> Result<()> {
             model: None,
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })
@@ -262,6 +263,7 @@ async fn resume_replays_permissions_messages() -> Result<()> {
             model: None,
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })
@@ -361,6 +363,7 @@ async fn resume_and_fork_append_permissions_messages() -> Result<()> {
             model: None,
             effort: None,
             summary: None,
+            service_tier: None,
             collaboration_mode: None,
             personality: None,
         })