[tui] Update Fast slash command description (#13458)

## Summary
- update the /fast slash command description to mention fastest
inference
- mention the 3X plan usage tradeoff in the help copy

## Testing
- cargo test -p codex-tui slash_command (currently blocked by an
unrelated latest-main codex-tui compile error in chatwidget.rs:
refresh_queued_user_messages missing)

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/tui/src/slash_command.rs

@@ -90,7 +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::Fast => "toggle Fast mode to enable fastest inference at 3X plan usage",
             SlashCommand::Personality => "choose a communication style for Codex",
             SlashCommand::Realtime => "toggle realtime voice mode (experimental)",
             SlashCommand::Settings => "configure realtime microphone/speaker",