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/tui/src/slash_command.rs

@@ -13,6 +13,7 @@ pub enum SlashCommand {
     // DO NOT ALPHA-SORT! Enum order is presentation order in the popup, so
     // more frequently used commands should be listed first.
     Model,
+    Fast,
     Approvals,
     Permissions,
     #[strum(serialize = "setup-default-sandbox")]
@@ -89,6 +90,7 @@ impl SlashCommand {
             SlashCommand::MemoryDrop => "DO NOT USE",
             SlashCommand::MemoryUpdate => "DO NOT USE",
             SlashCommand::Model => "choose what model and reasoning effort to use",
+            SlashCommand::Fast => "toggle Fast mode for supported models",
             SlashCommand::Personality => "choose a communication style for Codex",
             SlashCommand::Realtime => "toggle realtime voice mode (experimental)",
             SlashCommand::Settings => "configure realtime microphone/speaker",
@@ -123,6 +125,7 @@ impl SlashCommand {
             SlashCommand::Review
                 | SlashCommand::Rename
                 | SlashCommand::Plan
+                | SlashCommand::Fast
                 | SlashCommand::SandboxReadRoot
         )
     }
@@ -137,6 +140,7 @@ impl SlashCommand {
             | SlashCommand::Compact
             // | SlashCommand::Undo
             | SlashCommand::Model
+            | SlashCommand::Fast
             | SlashCommand::Personality
             | SlashCommand::Approvals
             | SlashCommand::Permissions