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/agent_test.go

@@ -0,0 +1,71 @@
+// 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 deviceagent
+
+import (
+	"fmt"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestAgent_currentHostInfo(t *testing.T) {
+	t.Parallel()
+
+	t.Run(
+		"reuses cached host info before refresh interval",
+		func(t *testing.T) {
+			t.Parallel()
+
+			count := 0
+			a := &Agent{
+				collectHostInfo: func() HostInfo {
+					count++
+					return HostInfo{Hostname: fmt.Sprintf("host-%d", count)}
+				},
+			}
+
+			now := time.Unix(1_000, 0)
+			first := a.currentHostInfo(now)
+			second := a.currentHostInfo(now.Add(2 * time.Hour))
+
+			assert.Equal(t, 1, count)
+			assert.Equal(t, first, second)
+		},
+	)
+
+	t.Run(
+		"refreshes host info when cache expires",
+		func(t *testing.T) {
+			t.Parallel()
+
+			count := 0
+			a := &Agent{
+				collectHostInfo: func() HostInfo {
+					count++
+					return HostInfo{Hostname: fmt.Sprintf("host-%d", count)}
+				},
+			}
+
+			now := time.Unix(2_000, 0)
+			first := a.currentHostInfo(now)
+			second := a.currentHostInfo(now.Add(hostInfoRefreshInterval + time.Minute))
+
+			assert.Equal(t, 2, count)
+			assert.NotEqual(t, first, second)
+		},
+	)
+}