v0.5.1: live-validation prep — fix unmounted-dir durable_id mis-id + watchdog UUID memory

Surfaced preparing the live USB validation on demo-felhom:
- observe.go: an unmounted removable dir-storage no longer falls through to the ROOT fs
  for its backing device/UUID — durable_id was becoming uuid:<root-uuid> (a DR mis-id that
  would re-attach the wrong disk). Now derived only from the target's own mountpoint;
  unmounted → no device + stable store:<name> durable_id. Removed containingMountDevice.
- watchdog.go: remember the fs-UUID observed while attached and backfill it onto the
  re-mount target, so re-mount works even if the known-set cache refreshed mid-drop
  (doc 03 §7 "sourced from the existing definition").

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/felhom-agent/main.go

@@ -31,7 +31,7 @@ import (
 
 // version is the agent version. Overridable at build time with
 // -ldflags "-X main.version=<v>"; defaults to the in-repo CHANGELOG version.
-var version = "0.5.0"
+var version = "0.5.1"
 
 func main() {
 	var (