Add probo-agent CLI and deviceagent library

Introduce the standalone device agent binary and shared library
for enrollment, posture checks, self-update, and OS service
integration. Include build targets, module deps, and release
workflow so the agent can ship independently of server changes.

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/deviceagent/update/archive.go

@@ -0,0 +1,151 @@
+// Copyright (c) 2025-2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package update
+
+import (
+	"archive/tar"
+	"archive/zip"
+	"compress/gzip"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"path"
+	"path/filepath"
+)
+
+const (
+	maxExtractedFileSize = 200 * 1024 * 1024 // 200 MiB hard cap per file
+)
+
+// extractBinary extracts the agent binary at
+// `<ArchiveDir>/<BinaryName>` from archivePath into workDir and
+// returns the absolute path of the written binary.
+func extractBinary(archivePath string, layout AssetLayout, workDir string) (string, error) {
+	wantPath := path.Join(layout.ArchiveDir, layout.BinaryName)
+	dest := filepath.Join(workDir, "probo-agent.new")
+
+	if layout.IsZip {
+		if err := extractZipFile(archivePath, wantPath, dest); err != nil {
+			return "", err
+		}
+	} else {
+		if err := extractTarGzFile(archivePath, wantPath, dest); err != nil {
+			return "", err
+		}
+	}
+
+	if _, err := os.Stat(dest); err != nil {
+		return "", fmt.Errorf("update: extracted binary missing: %w", err)
+	}
+
+	return dest, nil
+}
+
+func extractTarGzFile(archivePath, wantPath, dest string) error {
+	f, err := os.Open(archivePath)
+	if err != nil {
+		return fmt.Errorf("cannot open archive: %w", err)
+	}
+	defer func() { _ = f.Close() }()
+
+	gz, err := gzip.NewReader(f)
+	if err != nil {
+		return fmt.Errorf("cannot read gzip: %w", err)
+	}
+	defer func() { _ = gz.Close() }()
+
+	tr := tar.NewReader(gz)
+	for {
+		hdr, err := tr.Next()
+		if errors.Is(err, io.EOF) {
+			break
+		}
+
+		if err != nil {
+			return fmt.Errorf("cannot read tar entry: %w", err)
+		}
+
+		if path.Clean(hdr.Name) != wantPath {
+			continue
+		}
+
+		if hdr.Typeflag != tar.TypeReg && hdr.Typeflag != tar.TypeRegA {
+			return fmt.Errorf("update: %s is not a regular file", wantPath)
+		}
+
+		return writeStream(dest, tr, 0o755)
+	}
+
+	return fmt.Errorf("update: %s missing from archive", wantPath)
+}
+
+func extractZipFile(archivePath, wantPath, dest string) error {
+	r, err := zip.OpenReader(archivePath)
+	if err != nil {
+		return fmt.Errorf("cannot open zip: %w", err)
+	}
+	defer func() { _ = r.Close() }()
+
+	for _, f := range r.File {
+		if path.Clean(f.Name) != wantPath {
+			continue
+		}
+
+		if f.FileInfo().IsDir() {
+			return fmt.Errorf("update: %s is a directory", wantPath)
+		}
+
+		rc, err := f.Open()
+		if err != nil {
+			return fmt.Errorf("cannot open %s in zip: %w", wantPath, err)
+		}
+
+		err = writeStream(dest, rc, 0o755)
+		_ = rc.Close()
+
+		return err
+	}
+
+	return fmt.Errorf("update: %s missing from archive", wantPath)
+}
+
+func writeStream(dest string, src io.Reader, mode os.FileMode) error {
+	out, err := os.OpenFile(dest, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, mode)
+	if err != nil {
+		return fmt.Errorf("cannot create %s: %w", dest, err)
+	}
+
+	if _, err := io.Copy(out, io.LimitReader(src, maxExtractedFileSize+1)); err != nil {
+		_ = out.Close()
+		return fmt.Errorf("cannot write %s: %w", dest, err)
+	}
+
+	if err := out.Close(); err != nil {
+		return fmt.Errorf("cannot close %s: %w", dest, err)
+	}
+
+	stat, err := os.Stat(dest)
+	if err != nil {
+		return fmt.Errorf("cannot stat %s: %w", dest, err)
+	}
+
+	if stat.Size() > maxExtractedFileSize {
+		_ = os.Remove(dest)
+		return fmt.Errorf("update: extracted file exceeds %d bytes", maxExtractedFileSize)
+	}
+
+	return nil
+}