#8440: Docs: add cron troubleshooting for multi-gateway + nextWakeAtMs

This prevents a common failure mode where users assume nextWakeAtMs: null means the scheduler is dead, when it usually means no enabled future job (often due to multi-gateway confusion). • Adds a short “Troubleshooting (fast)” section to docs/automation/cron-jobs.md. • Covers two common pitfalls: 1. multi-profile / multi-Gateway confusion (storePath per gateway) 2. nextWakeAtMs: null meaning no eligible enabled future job (not a dead scheduler) • Tested: docs-only change. AI-assisted: yes (content derived from real debugging session).