Add Alertmanager silence during stack deployments

Commit `592ac8e`

startover.sh: Create 30-minute silence before full deployment, add colored logging, error handling for each stack
swarm.md skill: Add instructions to create 15-minute silence before any stack deployment to prevent alert spam

Silences automatically expire after the configured duration.

Co-Authored-By: Claude Opus 4.5 noreply@anthropic.com