Decommission LXC 109 (syncthing)

Container had been stopped since 2026-04-21 and was never re-enabled.
pct destroy 109 --purge cleaned up vm-109-disk-0 on local-lvm and the
config file. /mnt/library/syncthing subtree was already empty at the
time of destruction and is retained as an empty dir (no real data to
migrate or back up).

- README.md, containers/index.md: removed row, moved to "recently
  destroyed" table
- hosts/hubris.md: dropped from /mnt/library subtree list, updated
  containers/index summary line, added changelog entry
- infrastructure/media-permissions.md: dropped from membership table
  and onboarding example, generalised pct-exec gotcha hostname,
  added changelog
- vms/100-zimaos.md: dropped from "existing fleet" enumeration
- containers/102-nfs-export.md: dropped from bind-mount sibling list
  (7 LXCs now, not 8)
- containers/109-syncthing.md: deleted

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

vms/100-zimaos.md

@@ -1,6 +1,6 @@
 # 100 — `zimaos`
 
-ZimaOS (IceWhale / CasaOS-family NAS distro), installed as a Proxmox VM to evaluate it as a potential primary NAS frontend in front of `/mnt/library` — alongside the existing fleet ([nextcloud (114)](../containers/114-nextcloud.md), [jellyfin (101)](../containers/101-jellyfin.md), [syncthing (109)](../containers/109-syncthing.md), [mule-images (120)](../containers/120-mule-images.md)).
+ZimaOS (IceWhale / CasaOS-family NAS distro), installed as a Proxmox VM to evaluate it as a potential primary NAS frontend in front of `/mnt/library` — alongside the existing fleet ([nextcloud (114)](../containers/114-nextcloud.md), [jellyfin (101)](../containers/101-jellyfin.md), [mule-images (120)](../containers/120-mule-images.md)).
 
 ## At a glance
 - **Type:** QEMU VM