Instead of dropping a fallback-only grok-cli pair when no GROK_API_KEY is
Fusion-visible, defer it: the session runs the configured primary, and on
the first retryable model-selection failure it creates a session on the
Grok CLI runtime with the fallback model and re-issues the failed prompt
there; later prompts stay on the swapped session. Engagement reports
through onFallbackModelUsed and an ids-only
session:grok-cli-fallback-engaged run-audit event. The pair is dropped
(with audit flag) only when the Grok runtime plugin is unavailable.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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