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/_laptop-template

@@ -0,0 +1,5 @@
+# host: TEMPLATE for the laptop.  Copy to rules/hosts/<hostname> on that machine
+# (or edit this repo there, since rules/ itself travels in git).
+# Deliberately NOT including: media, misc, and everything under the denylist.
+base
+docs