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.timer

@@ -0,0 +1,12 @@
+[Unit]
+Description=homehub freshness check (half-hourly; the only thing that may complain)
+
+[Timer]
+OnBootSec=2min
+OnUnitActiveSec=30min
+RandomizedDelaySec=2min
+AccuracySec=1min
+Persistent=true
+
+[Install]
+WantedBy=timers.target