Add Phase 1 remote client registry and heartbeat agent

- add remote client register, heartbeat and list endpoints
- add remote client repository and service
- add minimal macOS remote agent with config, register and heartbeat loop
- keep client_id as leading identity
- keep status fields separated: last_seen, status, last_error, reachable_at
- avoid changes to local storage flows, PathGuard and /Volumes behavior

finder_commander/remote_client_agent.launchd.json

@@ -0,0 +1,15 @@
+{
+  "agent_access_token": "change-me-agent-token",
+  "client_id": "",
+  "display_name": "MacBook Pro van Jan",
+  "endpoint": "http://192.168.1.25:8765",
+  "heartbeat_interval_seconds": 20,
+  "platform": "macos",
+  "registration_token": "change-me-registration-token",
+  "shares": {
+    "downloads": "/Users/jan/Downloads",
+    "movies": "/Users/jan/Movies",
+    "pictures": "/Users/jan/Pictures"
+  },
+  "webmanager_base_url": "http://127.0.0.1:8080"
+}