SkyrimNet integration is retired; references in architecture docs were
rotting. Removes "SkyrimNet" mentions from runtime-engine and
inference-and-memory architecture docs (replaced with generic "multi-agent
failure-modes" / "Mantella-class failure-modes"), and from
development-conventions ("old nimmersky workloads").
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>