Add /fast for Codex models that support priority routing

[bikouz]

Summary

• Add a session-scoped /fast slash command that toggles Fast mode for models that advertise a Fast service tier (Codex catalog service_tiers / legacy additional_speed_tiers).
• Parse Codex service tiers into model metadata and expose them over ACP so the pager can gate the command on the active model.
• Carry the selected service tier through sampling config into Responses requests as service_tier=priority when Fast is enabled; clear it for standard routing.
• Preserve the session reasoning effort while toggling Fast mode, and show clear enabled/disabled feedback.

Behavior

• /fast appears only when the current model supports Fast.
• Enable maps to wire value priority; disable uses standard routing (omitted service_tier).
• Model switches drop a previously selected tier when the new model does not support it.

Notes

• Covers the full path from Codex catalog parse through shell model switch and pager dispatch.