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/status.go

@@ -0,0 +1,44 @@
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/portainer/kubesolo-os/update/pkg/grubenv"
+)
+
+// Status displays the current A/B slot configuration and boot state.
+func Status(args []string) error {
+	opts := parseOpts(args)
+	env := grubenv.New(opts.GrubenvPath)
+
+	vars, err := env.ReadAll()
+	if err != nil {
+		return fmt.Errorf("reading GRUB environment: %w", err)
+	}
+
+	activeSlot := vars["active_slot"]
+	bootCounter := vars["boot_counter"]
+	bootSuccess := vars["boot_success"]
+
+	passiveSlot := "B"
+	if activeSlot == "B" {
+		passiveSlot = "A"
+	}
+
+	fmt.Println("KubeSolo OS — A/B Partition Status")
+	fmt.Println("───────────────────────────────────")
+	fmt.Printf("  Active slot:   %s\n", activeSlot)
+	fmt.Printf("  Passive slot:  %s\n", passiveSlot)
+	fmt.Printf("  Boot counter:  %s\n", bootCounter)
+	fmt.Printf("  Boot success:  %s\n", bootSuccess)
+
+	if bootSuccess == "1" {
+		fmt.Println("\n  ✓ System is healthy (boot confirmed)")
+	} else if bootCounter == "0" {
+		fmt.Println("\n  ✗ Boot counter exhausted — rollback will occur on next reboot")
+	} else {
+		fmt.Printf("\n  ⚠ Boot pending verification (%s attempts remaining)\n", bootCounter)
+	}
+
+	return nil
+}