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>

build/scripts/inject-kubesolo.sh

@@ -73,6 +73,16 @@ else
     echo "    WARN: Cloud-init binary not found (run 'make build-cloudinit' to build)"
 fi
 
+# Update agent binary (Go, built separately)
+UPDATE_BIN="$CACHE_DIR/kubesolo-update"
+if [ -f "$UPDATE_BIN" ]; then
+    cp "$UPDATE_BIN" "$ROOTFS/usr/lib/kubesolo-os/kubesolo-update"
+    chmod +x "$ROOTFS/usr/lib/kubesolo-os/kubesolo-update"
+    echo "    Installed update agent ($(du -h "$UPDATE_BIN" | cut -f1))"
+else
+    echo "    WARN: Update agent not found (run 'make build-update-agent' to build)"
+fi
+
 # --- 3. Kernel modules list ---
 cp "$PROJECT_ROOT/build/config/modules.list" "$ROOTFS/usr/lib/kubesolo-os/modules.list"