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.

rules/sets.d/docs.list

@@ -0,0 +1,2 @@
+# set: docs — real documents. On the laptop too (decision D4).
+/Documents/