refactor(dotfiles): inline registry operations

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

@@ -15,3 +15,16 @@ Do not apply this rule when a binding names a local constant used to keep siblin
 
 Shared convenience bindings hide dependency direction.
 Explicit references show which value owns a concept and which value consumes it.
+
+## Do not use single-use merge wrappers
+
+### Rule
+
+Do not bind a local function that extends an attribute-set argument when it has one caller.
+Merge the fields in the caller.
+Keep the function when two or more callers use it.
+
+### Rationale
+
+Single-use wrappers hide construction of the result.
+Direct merges show the added fields and their owner.