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/launchd/com.webmanager.remote-client-agent.plist

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+  <dict>
+    <key>Label</key>
+    <string>com.webmanager.remote-client-agent</string>
+    <key>ProgramArguments</key>
+    <array>
+      <string>/usr/bin/python3</string>
+      <string>-u</string>
+      <string>/workspace/webmanager-mvp/finder_commander/remote_client_agent.py</string>
+      <string>--config</string>
+      <string>/workspace/webmanager-mvp/finder_commander/remote_client_agent.launchd.json</string>
+    </array>
+    <key>RunAtLoad</key>
+    <true/>
+    <key>KeepAlive</key>
+    <true/>
+    <key>StandardOutPath</key>
+    <string>/tmp/com.webmanager.remote-client-agent.out.log</string>
+    <key>StandardErrorPath</key>
+    <string>/tmp/com.webmanager.remote-client-agent.err.log</string>
+  </dict>
+</plist>

finder_commander/remote_client_agent.launchd.json

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+{
+  "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"
+}