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>
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