Adds a fastModeScope setting (both|openai|claude, default both). setFastMode(true)
derives the service tier from it (both->priority, openai->openai-only,
claude->claude-only) instead of hardcoding unscoped priority; the off path and
the already-on no-op are unchanged. /fast status now reports the active scope.
Default both preserves existing behavior.
metaphorics