docs(skills): refine nix let rules

docs/TODO.md

@@ -18,6 +18,7 @@
 - amazon / actualbudget exporter
 - vim-obsidian-markdown
 - dictionary cli tool
+- repo-local Nix style linter
 - kitchen.md
 
 # migration

src/dotfiles/default.nix

@@ -19,6 +19,15 @@ let
     root = root + "/modules";
   };
 
+  specialArgs = {
+    inherit
+      inputs
+      self
+      util
+      ;
+    registry = registry.collected;
+  };
+
   guestModules = import ./guests.nix {
     inherit
       inputs
@@ -30,15 +39,6 @@ let
     repositoryModules = modules;
     root = root + "/guests";
   };
-
-  specialArgs = {
-    inherit
-      inputs
-      self
-      util
-      ;
-    registry = registry.collected;
-  };
 in
 {
   formatter.${system} =

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

@@ -5,9 +5,11 @@
 ### Rule
 
 Do not add a `let` binding only to let sibling values share one expression.
+Apply this rule only when one sibling value clearly owns the concept.
 When one sibling value owns the concept, reference that sibling value directly.
 When a flake output owns the public value, reference that output through `self`.
 Do not make an output attrset recursive only to reference sibling outputs.
+Do not apply this rule when a binding names a local constant used to keep sibling assignments in sync.
 
 ### Rationale
 
@@ -25,4 +27,5 @@ Use forward references only for intentional recursion or fixed-point wiring.
 
 Nix permits recursive `let` bindings.
 Top-down dependency order makes ordinary wiring easier to read.
+This rule can become a linter rule because binding references are mechanically detectable.