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-update-agent.sh

@@ -3,15 +3,19 @@
 #
 # Builds a static Linux binary for the update agent.
 # Output: build/cache/kubesolo-update
+#
+# Environment:
+#   TARGET_ARCH  Target architecture (default: amd64, also supports: arm64)
 set -euo pipefail
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 PROJECT_ROOT="$(cd "$SCRIPT_DIR/../.." && pwd)"
 UPDATE_DIR="$PROJECT_ROOT/update"
 CACHE_DIR="$PROJECT_ROOT/build/cache"
+TARGET_ARCH="${TARGET_ARCH:-amd64}"
 OUTPUT="$CACHE_DIR/kubesolo-update"
 
-echo "=== Building KubeSolo Update Agent ==="
+echo "=== Building KubeSolo Update Agent (linux/$TARGET_ARCH) ==="
 
 # Ensure output dir exists
 mkdir -p "$CACHE_DIR"
@@ -22,7 +26,7 @@ echo "--- Running tests ---"
 
 # Build static binary
 echo "--- Compiling static binary ---"
-(cd "$UPDATE_DIR" && CGO_ENABLED=0 GOOS=linux GOARCH=amd64 \
+(cd "$UPDATE_DIR" && CGO_ENABLED=0 GOOS=linux GOARCH="$TARGET_ARCH" \
     go build -ldflags='-s -w' -o "$OUTPUT" .)
 
 SIZE=$(ls -lh "$OUTPUT" | awk '{print $5}')