feat: add distribution and fleet management — CI/CD, OCI, metrics, ARM64 (Phase 5)

- Gitea Actions CI pipeline: Go tests, build, shellcheck on push/PR
- Gitea Actions release pipeline: full build + artifact upload on version tags
- OCI container image builder for registry-based OS distribution
- Zero-dependency Prometheus metrics endpoint (kubesolo_os_info, boot,
  memory, update status) with 10 tests
- USB provisioning tool for air-gapped deployments with cloud-init injection
- ARM64 cross-compilation support (TARGET_ARCH env var + build-cross.sh)
- Updated build scripts to accept TARGET_ARCH for both amd64 and arm64
- New Makefile targets: oci-image, build-cross

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

update/cmd/metrics.go

@@ -0,0 +1,21 @@
+package cmd
+
+import (
+	"flag"
+	"fmt"
+
+	"github.com/portainer/kubesolo-os/update/pkg/metrics"
+)
+
+// Metrics starts the Prometheus-compatible metrics HTTP server.
+func Metrics(args []string) error {
+	fs := flag.NewFlagSet("metrics", flag.ExitOnError)
+	listenAddr := fs.String("listen", ":9100", "Metrics HTTP listen address")
+	grubenvPath := fs.String("grubenv", "/boot/grub/grubenv", "Path to grubenv file")
+	if err := fs.Parse(args); err != nil {
+		return fmt.Errorf("parse flags: %w", err)
+	}
+
+	srv := metrics.NewServer(*listenAddr, *grubenvPath)
+	return srv.ListenAndServe()
+}