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/misc.list

@@ -0,0 +1,7 @@
+# set: misc — small hand-written trees that are not git repos.
+# Anything here that later becomes a repo must be REMOVED from this set and picked
+# up by `homehub git inventory` instead (§6).
+/journal/
+/projects/
+/mu_code/
+/public_html/