Adds a paired `authRequiredMessage()` method to AgentProvider so per-provider auth-failure remediation can differ. Claude returns the Anthropic/`claude` instruction; future providers (Codex, OpenCode, …) can return their own remediation text. The poll-loop calls `provider.authRequiredMessage?.()` and falls back to a generic message if a provider implements `isAuthRequired` without supplying its own remediation. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>