agent v0.37.0: host-reboot remount re-resolves enrolled drives by fs-UUID

TASK A — close out the reboot story (agent half). Root cause (pinned live on
felhom-pve): an enrolled .mount unit left `disabled` by a prior detach never
auto-mounts at boot, and kernel re-enumeration can move a drive's node
(/dev/sdb->sdc). Fix re-asserts every enrolled mount by filesystem UUID at
startup + on the periodic tick.

- ResolveStorageDevice: resolve uuid:<fs-uuid> -> current /dev node via
  /dev/disk/by-uuid (never a cached node); errors if absent.
- parseFelhomMountUnit: pure inverse of renderMountUnit (marker-gated).
- (*SudoHostOps).ReassertEnrolledMounts: re-run EnsureMount (enable --now) for
  any enrolled unit not in /proc/mounts; idempotent, skips mounted/absent.
- main.go: runs before ReassertGuestBinds at startup + on the 20s tick.
- tests (Linux, seam=device resolution): letter-move tolerated (sdb->sdc) +
  red-proof companion, absent/scheme rejection, render->parse round-trip.

TASK A2 verdict: enrolling a NEW drive does NOT need an LXC restart — the path
lands on the live AttachDrive (shared parent, named live slots), not RebootGuest.

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

internal/storage/mountunit.go

@@ -5,6 +5,41 @@ import (
 	"strings"
 )
 
+// felhomUnitMarker is the header renderMountUnit writes; parseFelhomMountUnit uses it to tell our
+// units apart from any other .mount unit on the host.
+const felhomUnitMarker = "Managed by felhom-agent"
+
+// parseFelhomMountUnit is the inverse of renderMountUnit for the fields the host-reboot re-assert needs.
+// It returns the MountSpec (Name from Description, UUID from What=/dev/disk/by-uuid/<UUID>, Where, Type,
+// Options), or ok=false when the content is not a felhom-rendered by-UUID mount unit. Pure → unit-tested.
+func parseFelhomMountUnit(content string) (MountSpec, bool) {
+	if !strings.Contains(content, felhomUnitMarker) {
+		return MountSpec{}, false
+	}
+	var spec MountSpec
+	for _, line := range strings.Split(content, "\n") {
+		line = strings.TrimSpace(line)
+		switch {
+		case strings.HasPrefix(line, "Description=Felhom storage mount "):
+			spec.Name = strings.TrimPrefix(line, "Description=Felhom storage mount ")
+		case strings.HasPrefix(line, "What="):
+			if u, found := strings.CutPrefix(strings.TrimPrefix(line, "What="), byUUIDDir+"/"); found {
+				spec.UUID = u
+			}
+		case strings.HasPrefix(line, "Where="):
+			spec.Where = strings.TrimPrefix(line, "Where=")
+		case strings.HasPrefix(line, "Type="):
+			spec.FSType = strings.TrimPrefix(line, "Type=")
+		case strings.HasPrefix(line, "Options="):
+			spec.Options = strings.TrimPrefix(line, "Options=")
+		}
+	}
+	if spec.UUID == "" || spec.Where == "" { // not a by-uuid mount we can re-resolve
+		return MountSpec{}, false
+	}
+	return spec, true
+}
+
 // renderMountUnit builds the systemd .mount unit content for a (already-validated) spec.
 // Keyed by fs-UUID via What=/dev/disk/by-uuid/<UUID> so it survives /dev/sdX renumbering;
 // WantedBy=multi-user.target so `enable` makes it persist across reboot.