Adds 4-layer inventory system as Single Source of Truth:
- inventory/physical.yaml: Physical hosts (Hestia, Proxmox, TrueNAS, ER605)
- inventory/proxmox.yaml: VMs and LXC containers (dockerino, media, homeassistant)
- inventory/truenas.yaml: ZFS pools, disks, datasets, NFS exports
- inventory/network.yaml: VLANs, subnets, DNS
All files have PLACEHOLDER fields to be filled with real data
using the discovery commands in inventory/README.md
