#937 Workflow failure surfaces only 'exhausting retries' + internal task id; root cause invisible via API

EARS SPEC: - When a workflow run fails because a task exhausted its retries, GET /api/v1/workflows/{id} shall include the failing task's name and a redacted final error class and message (no secrets, no workflow variable values). - The trace endpoint shall include a span for a task that failed before its first external call. FOUND 2026-10-15 by highway-mcp (runs 4a3ae581, 5945b467): error was {message: 'Task permanently failed after exhausting retries', task_id: <internal>} while the real cause (every LLM candidate 402/410, #936) was only in worker logs. A customer cannot diagnose this. Parent epic #913. Must respect logging-security memory (log metadata, never variable values).