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/checks/shared.go

@@ -0,0 +1,84 @@
+// 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 checks
+
+import (
+	"context"
+	"time"
+)
+
+// Check keys shared across OS implementations.
+const (
+	KeyDiskEncryption    = "DISK_ENCRYPTION"
+	KeyScreenLock        = "SCREEN_LOCK"
+	KeyFirewallEnabled   = "FIREWALL_ENABLED"
+	KeyTimeSync          = "TIME_SYNC"
+	KeyOSVersion         = "OS_VERSION"
+	KeyAutoUpdate        = "AUTO_UPDATE"
+	KeyPasswordPolicy    = "PASSWORD_POLICY"
+	KeyRemoteLogin       = "REMOTE_LOGIN"
+	KeyMalwareProtection = "MALWARE_PROTECTION"
+)
+
+type funcCheck struct {
+	key string
+	run func(ctx context.Context) Result
+}
+
+func (c funcCheck) Key() string { return c.key }
+
+func (c funcCheck) Run(ctx context.Context) Result {
+	r := c.run(ctx)
+	if r.CheckKey == "" {
+		r.CheckKey = c.key
+	}
+	if r.ObservedAt.IsZero() {
+		r.ObservedAt = time.Now().UTC()
+	}
+	return r
+}
+
+func pass(ev map[string]any) Result {
+	return Result{Status: StatusPass, Evidence: ev}
+}
+
+func fail(ev map[string]any) Result {
+	return Result{Status: StatusFail, Evidence: ev}
+}
+
+func unknown(ev map[string]any) Result {
+	return Result{Status: StatusUnknown, Evidence: ev}
+}
+
+func notApplicable(ev map[string]any) Result {
+	return Result{Status: StatusNotApplicable, Evidence: ev}
+}
+
+// truncate limits oversized evidence values.
+func truncate(s string, n int) string {
+	if len(s) <= n {
+		return s
+	}
+	return s[:n] + "…"
+}
+
+// errString returns "" for a nil error.
+func errString(err error) string {
+	if err == nil {
+		return ""
+	}
+
+	return err.Error()
+}