v0.5.0: slice 5 Phase B — the host-root surface (mounts + SMART + grow + destructive gate)

The privileged write surface, isolated behind a narrow, arg-validated, adversarially-
tested seam (HostOps), the same discipline as the slice-4 gate. Completes slice 5.

- internal/storage: HostOps seam + SudoHostOps (systemd .mount units by fs-UUID, detach,
  SMART, lvs) via sudoers allowlist + fixed arg vectors, no shell; NoopHostOps fallback.
- validate.go: strict UUID/mount-path/device/LVM validators + in-process systemd-escape.
  Headline test: adversarial matrix (metacharacters/traversal/malformed) refused with
  zero exec.
- smart.go: smartctl SATA + NVMe parse, UNKNOWN-degrade; lvs thin-pool metadata fill.
- observer enrichment (Observe only): fills smart + thin-pool metadata.
- watchdog: benign re-mount response off the poll path (DevicePresent probe, rate-limited).
- reconcile: ActionResize (benign, grow-only) + proxmox.ResizeLXC; destructive storage ops
  (ClassStorageWipe/Decommission) through the slice-4 gate, target-scoped; built+tested,
  inert live.
- --selftest=storage [-watch] live harness; configs/felhom-agent.sudoers; privileged.* knobs.

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

internal/storage/mountunit.go

@@ -0,0 +1,44 @@
+package storage
+
+import (
+	"fmt"
+	"strings"
+)
+
+// 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.
+//
+// All interpolated values are pre-validated by the caller (ValidateUUID / ValidateMountPath
+// / validateUnitOpt), so no value here can carry a newline or inject an extra directive.
+func renderMountUnit(spec MountSpec) string {
+	what := byUUIDDir + "/" + spec.UUID
+	var b strings.Builder
+	b.WriteString("# Managed by felhom-agent — do not edit by hand.\n")
+	b.WriteString("[Unit]\n")
+	fmt.Fprintf(&b, "Description=Felhom storage mount %s\n", sanitizeDesc(spec.Name))
+	b.WriteString("After=local-fs-pre.target\n")
+	b.WriteString("\n[Mount]\n")
+	fmt.Fprintf(&b, "What=%s\n", what)
+	fmt.Fprintf(&b, "Where=%s\n", spec.Where)
+	if spec.FSType != "" {
+		fmt.Fprintf(&b, "Type=%s\n", spec.FSType)
+	}
+	if spec.Options != "" {
+		fmt.Fprintf(&b, "Options=%s\n", spec.Options)
+	}
+	b.WriteString("\n[Install]\n")
+	b.WriteString("WantedBy=multi-user.target\n")
+	return b.String()
+}
+
+// sanitizeDesc keeps the Description line single-line and harmless (it is cosmetic; the
+// name is already a Proxmox storage id, but be defensive against any newline).
+func sanitizeDesc(name string) string {
+	name = strings.ReplaceAll(name, "\n", " ")
+	name = strings.ReplaceAll(name, "\r", " ")
+	if name == "" {
+		return "(unnamed)"
+	}
+	return name
+}