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/app-server/src/codex_message_processor.rs

@@ -5323,6 +5323,7 @@ impl CodexMessageProcessor {
                 model,
                 effort,
                 summary: Some(summary),
+                service_tier: None,
                 final_output_json_schema: output_schema,
                 collaboration_mode: None,
                 personality: None,
@@ -5880,6 +5881,7 @@ impl CodexMessageProcessor {
                     model: params.model,
                     effort: params.effort.map(Some),
                     summary: params.summary,
+                    service_tier: None,
                     collaboration_mode,
                     personality: params.personality,
                 })