docs: field lesson — bridge re-enslave + wedged nconnect NFS recovery [#780]

https://projects.knownelement.com/issues/769#note-4152

AGENTS.md

@@ -825,3 +825,11 @@ All work is tracked in [Redmine](https://projects.knownelement.com)
 - **PBS 4.x gotchas:** prune retention lives in `prune-job` (datastore keep-*
   opts are gone); API tokens need their OWN acl entry — user-only ACL yields
   "Cannot find datastore" from PVE.
+- **PVE bridge port + hotplug NIC (#780):** a re-registered NIC (USB move
+  changes its enumeration path) comes back UP but is NOT re-enslaved to its
+  bridge — `ifup <nic>` is not enough; run `ifup <bridge>` and confirm
+  `bridge link` shows `state forwarding`. NFS `hard` mounts mostly self-heal,
+  but `nconnect=4` mounts can stay wedged on stale channels after a flap:
+  force-remount (`umount -f -l`, recreate the mountpoint dir if the lazy
+  umount removed it, fresh mount). `pvesm status` "active" can be stale —
+  verify with a real read, not the status line.