Add .rules: invariants and gotchas for future development

Most entries are a data-loss bug that was found in testing and must not come
back: filter ordering, harvest-before-pull, seen-gated deletion attribution,
edits-beat-deletes, POSIX sh on DSM, and the never-2>/dev/null-an-rsync rule.
Symlinked as AGENTS.md so a future agent session loads it automatically.

README.md

@@ -13,6 +13,7 @@ is only the operational half.
   lib/merge_bookmarks.py       class C union-merge (jq + python3 only)
   systemd/homehub-*.{service,timer}
   DESIGN.md                  rationale for everything below
+  .rules -> AGENTS.md          invariants + gotchas for future changes (auto-loaded by agents)
 ```
 
 ## Install (per machine)