feat: add A/B partition updates with GRUB and Go update agent (Phase 3)

Implement atomic OS updates via A/B partition scheme with automatic
rollback. GRUB bootloader manages slot selection with a 3-attempt
boot counter that auto-rolls back on repeated health check failures.

GRUB boot config:
- A/B slot selection with boot_counter/boot_success env vars
- Automatic rollback when counter reaches 0 (3 failed boots)
- Debug, emergency shell, and manual slot-switch menu entries

Disk image (refactored):
- 4-partition GPT layout: EFI + System A + System B + Data
- GRUB EFI/BIOS installation with graceful fallbacks
- Both system partitions populated during image creation

Update agent (Go, zero external deps):
- pkg/grubenv: read/write GRUB env vars (grub-editenv + manual fallback)
- pkg/partition: find/mount/write system partitions by label
- pkg/image: HTTP download with SHA256 verification
- pkg/health: post-boot checks (containerd, API server, node Ready)
- 6 CLI commands: check, apply, activate, rollback, healthcheck, status
- 37 unit tests across all 4 packages

Deployment:
- K8s CronJob for automatic update checks (every 6 hours)
- ConfigMap for update server URL
- Health check Job for post-boot verification

Build pipeline:
- build-update-agent.sh compiles static Linux binary (~5.9 MB)
- inject-kubesolo.sh includes update agent in initramfs
- Makefile: build-update-agent, test-update-agent, test-update targets

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

update/cmd/rollback.go

@@ -0,0 +1,36 @@
+package cmd
+
+import (
+	"fmt"
+	"log/slog"
+
+	"github.com/portainer/kubesolo-os/update/pkg/grubenv"
+)
+
+// Rollback forces an immediate switch to the other partition.
+// Use this to manually revert to the previous version.
+func Rollback(args []string) error {
+	opts := parseOpts(args)
+	env := grubenv.New(opts.GrubenvPath)
+
+	activeSlot, err := env.ActiveSlot()
+	if err != nil {
+		return fmt.Errorf("reading active slot: %w", err)
+	}
+
+	passiveSlot, err := env.PassiveSlot()
+	if err != nil {
+		return fmt.Errorf("reading passive slot: %w", err)
+	}
+
+	slog.Info("forcing rollback", "from", activeSlot, "to", passiveSlot)
+
+	if err := env.ForceRollback(); err != nil {
+		return fmt.Errorf("rollback failed: %w", err)
+	}
+
+	fmt.Printf("Rolled back: %s → %s\n", activeSlot, passiveSlot)
+	fmt.Println("Reboot to complete rollback.")
+
+	return nil
+}