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Backport upstream Shannon PR #335: - Add credential validation activity that drives a real browser login before the full pipeline, catching bad credentials early - New email_login credentials type for magic-link and email-OTP flows - Make credentials.password optional for passwordless flows - Playwright stealth config (chrome.runtime, plugin simulation, UA) - Centralize prompt directory resolution into resolvePromptDir helper - New AUTH_LOGIN_FAILED error code with non-retryable classification - Remove dangerous-pattern validation on credentials.password - Pipeline-testing stub for auth validation (returns success) - Auth validation timeout of 10 minutes for browser-based login - .playwright directory workspace overlay for CLI/Docker Co-Authored-By: Paperclip <noreply@paperclip.ing>