docs(nix): explain guest secret access

src/skills/style/nix/rules/dotfiles.md

@@ -39,5 +39,5 @@ Each guest service group accesses its secrets through a read-only bind mount.
 
 ### Rationale
 
-SOPS materializes secrets on the host filesystem.
+Guest service accounts do not exist on the host, where SOPS creates secrets.
-Read-only bind mounts prevent guest services from modifying the host copy.
+The shared service group preserves read access across the guest boundary.