Add single-node demo inventory

[aronchick]

Summary

• add a generic one-node SSH inventory for demo-edge-01
• document the fastest non-Proxmox demo path for an existing VM or bare-metal host
• include validation and deploy commands using the existing expanso-edge role

Verification

• /tmp/jetpack-ref/target/debug/jetpack inventory-check --inventory deploy/inventory.single-node:deploy/secrets.example
• /tmp/jetpack-ref/target/debug/jetpack full-check --playbook deploy/playbooks/expanso-edge.yml --inventory deploy/inventory.single-node:deploy/secrets.example --roles deploy/roles
• git diff --check