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-sync.service

@@ -0,0 +1,19 @@
+[Unit]
+Description=homehub home-drive sync (rsync to NAS hub, mtime-arbitrated)
+After=network-online.target
+Wants=network-online.target
+Conflicts=sleep.target
+
+[Service]
+Type=oneshot
+# A background home sync must never make the machine stutter.
+Nice=19
+IOSchedulingClass=idle
+# Silent on success is a requirement, not an oversight (DESIGN.md §10).
+Environment=HH_QUIET=1
+ExecStart=%h/homehub/bin/homehub sync --exec
+TimeoutStartSec=3h
+# No retry storm: the next timer tick is the retry.
+
+[Install]
+WantedBy=default.target