homehub: rsync-based home-drive sync via Synology hub

DESIGN.md is the rationale; README.md the operations half.
Four data classes: files (mtime-arbitrated two-way), git repos (manifest +
clone, never byte-synced), the browser link bar (union merge), and everything
else denied by default.

systemd/homehub-fresh.service

@@ -0,0 +1,12 @@
+[Unit]
+Description=homehub freshness check: alerts only when a live machine has not synced for 24 h
+
+[Service]
+Type=oneshot
+# HH_QUIET is deliberately NOT set: the ok/stale lines belong in the journal, and
+# alert_send() uses DISPLAY+notify-send to reach the desktop when there is one.
+ExecStart=%h/homehub/bin/homehub fresh
+TimeoutStartSec=5min
+
+[Install]
+WantedBy=default.target