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>

build/scripts/build-cloudinit.sh

@@ -1,15 +1,19 @@
 #!/bin/bash
 # build-cloudinit.sh — Compile the cloud-init binary as a static Linux binary
+#
+# Environment:
+#   TARGET_ARCH  Target architecture (default: amd64, also supports: arm64)
 set -euo pipefail
 
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 PROJECT_ROOT="$(cd "$SCRIPT_DIR/../.." && pwd)"
 CACHE_DIR="${CACHE_DIR:-$PROJECT_ROOT/build/cache}"
 CLOUDINIT_SRC="$PROJECT_ROOT/cloud-init"
+TARGET_ARCH="${TARGET_ARCH:-amd64}"
 
 OUTPUT="$CACHE_DIR/kubesolo-cloudinit"
 
-echo "==> Building cloud-init binary..."
+echo "==> Building cloud-init binary (linux/$TARGET_ARCH)..."
 
 if ! command -v go >/dev/null 2>&1; then
     echo "ERROR: Go is not installed. Install Go 1.22+ to build cloud-init."
@@ -28,9 +32,9 @@ go test ./... -count=1 || {
     exit 1
 }
 
-# Build static binary for Linux amd64
-echo "    Compiling (CGO_ENABLED=0 GOOS=linux GOARCH=amd64)..."
-CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build \
+# Build static binary
+echo "    Compiling (CGO_ENABLED=0 GOOS=linux GOARCH=$TARGET_ARCH)..."
+CGO_ENABLED=0 GOOS=linux GOARCH="$TARGET_ARCH" go build \
     -ldflags='-s -w' \
     -o "$OUTPUT" \
     ./cmd/