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/hosts/g9

@@ -0,0 +1,6 @@
+# host: g9  (desktop, always at home, no disk-space pressure)
+# One set name per line; must match a file in rules/sets.d/
+base
+docs
+media
+misc