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/service/service_linux.go

@@ -0,0 +1,97 @@
+// 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 service
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+	"text/template"
+)
+
+const (
+	systemdUnitPath = "/etc/systemd/system/probo-agent.service"
+)
+
+const systemdUnitTmpl = `[Unit]
+Description=Probo device posture agent
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+ExecStart={{.ExePath}} run --dir {{.Dir}}
+Restart=always
+RestartSec=10
+# 75 is the exit code emitted after a successful self-update.
+# Treat it as a normal exit so the unit restarts without entering
+# the "failed" state.
+SuccessExitStatus=75
+User=root
+NoNewPrivileges=true
+PrivateTmp=true
+ProtectSystem=full
+
+[Install]
+WantedBy=multi-user.target
+`
+
+func Install(cfg Config) error {
+	if cfg.ExePath == "" {
+		return errors.New("executable path is required")
+	}
+
+	if cfg.Dir == "" {
+		return errors.New("state directory is required")
+	}
+
+	tmpl, err := template.New("unit").Parse(systemdUnitTmpl)
+	if err != nil {
+		return fmt.Errorf("cannot parse unit template: %w", err)
+	}
+
+	f, err := os.OpenFile(systemdUnitPath, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o644)
+	if err != nil {
+		return fmt.Errorf("cannot write systemd unit (need root?): %w", err)
+	}
+
+	defer func() { _ = f.Close() }()
+	if err := tmpl.Execute(f, cfg); err != nil {
+		return fmt.Errorf("cannot render systemd unit: %w", err)
+	}
+
+	if out, err := exec.Command("systemctl", "daemon-reload").CombinedOutput(); err != nil {
+		return fmt.Errorf("cannot run systemctl daemon-reload: %w: %s", err, strings.TrimSpace(string(out)))
+	}
+
+	if out, err := exec.Command("systemctl", "enable", "--now", "probo-agent.service").CombinedOutput(); err != nil {
+		return fmt.Errorf("cannot run systemctl enable --now: %w: %s", err, strings.TrimSpace(string(out)))
+	}
+
+	return nil
+}
+
+func Uninstall(cfg Config) error {
+	_ = exec.Command("systemctl", "disable", "--now", "probo-agent.service").Run()
+	if err := os.Remove(systemdUnitPath); err != nil && !errors.Is(err, os.ErrNotExist) {
+		return fmt.Errorf("cannot remove systemd unit: %w", err)
+	}
+
+	_ = exec.Command("systemctl", "daemon-reload").Run()
+
+	return nil
+}