When an agent turn fails, users only saw "요청을 완료하지 못했습니다. 다시
시도해 주세요." with no clue why. Capture the failure reason (explicit
error field, or provider-error text like "API Error: 529 Overloaded"
classified by detectClaudeProviderFailureMessage) in the turn controller
and append it to the failure message ("...\n\n오류 내용: <reason>",
secret-redacted + length-capped). Update the paired-room loop filter to
startsWith so a failure-with-reason is still recognized and not
re-injected into prompt history. Unit-tested.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
