decommission: destroy plato (LXC 126)

LXC 126 stopped and destroyed on hubris. Remove all live references:
inventory, container doc, host file, README, containers index, auto-deploy
pipeline, DNS entry, SSH access table, nfs-export mount list.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infrastructure/ssh-access.md

@@ -131,7 +131,6 @@ are managed by `ssh/deploy-keys.sh`. SSH user is `root`.
 | 120 | mule-images | `192.168.8.136` | photo-management |
 | 121 | caddy | `192.168.8.175` | reverse-proxy |
 | 122 | arriman | `192.168.8.132` | arr-stack |
-| 126 | plato | `192.168.8.190` | app |
 
 ### Workstations