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/registry.go

@@ -0,0 +1,55 @@
+// 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"
+	"sort"
+	"sync"
+)
+
+var (
+	registryMu sync.Mutex
+	registry   []Check
+)
+
+// Register adds a check implementation to the process registry.
+func Register(key string, run func(context.Context) Result) {
+	registryMu.Lock()
+	defer registryMu.Unlock()
+	registry = append(
+		registry,
+		funcCheck{
+			key: key,
+			run: run,
+		},
+	)
+}
+
+// All returns a stable snapshot of registered checks.
+func All() []Check {
+	registryMu.Lock()
+	defer registryMu.Unlock()
+
+	out := make([]Check, len(registry))
+	copy(out, registry)
+	sort.SliceStable(
+		out,
+		func(i, j int) bool {
+			return out[i].Key() < out[j].Key()
+		},
+	)
+	return out
+}