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/main.go

@@ -10,6 +10,7 @@
 //	kubesolo-update status         Show current A/B slot and boot status
 //	kubesolo-update sign           Sign update artifacts with Ed25519 key
 //	kubesolo-update genkey         Generate new Ed25519 signing key pair
+//	kubesolo-update metrics        Start Prometheus-compatible metrics server
 package main
 
 import (
@@ -48,6 +49,8 @@ func main() {
 		err = cmd.Sign(os.Args[2:])
 	case "genkey":
 		err = cmd.GenKey(os.Args[2:])
+	case "metrics":
+		err = cmd.Metrics(os.Args[2:])
 	default:
 		fmt.Fprintf(os.Stderr, "unknown command: %s\n\n", os.Args[1])
 		usage()
@@ -72,6 +75,7 @@ Commands:
   status       Show current A/B slot and boot status
   sign         Sign artifacts with Ed25519 private key (build system)
   genkey       Generate new Ed25519 signing key pair
+  metrics      Start Prometheus-compatible metrics HTTP server
 
 Options:
   --server URL     Update server URL (default: from /etc/kubesolo/update.conf)