docs(skills): add nix let review rule

src/skills/nix-style/SKILL.md

@@ -9,12 +9,8 @@ Use this skill as the source of truth for repository Nix conventions.
 
 ## Load rules
 
-Read every Markdown file under [`rules/`](rules/) before applying this skill.
-Discover rule files from the skill source:
-
-```bash
-find src/skills/nix-style/rules -maxdepth 1 -type f -name '*.md' -print | sort
-```
+Read every Markdown file directly under [`rules/`](rules/) before applying this skill.
+Resolve [`rules/`](rules/) relative to this `SKILL.md`.
 
 Finish when all discovered rule files have been read.
 
@@ -29,7 +25,8 @@ After editing Nix files, run:
 nix run .#fix
 ```
 
-Finish when the Nix change follows the loaded rules.
+Finish when `nix run .#fix` succeeds and the Nix change follows the loaded rules.
+If the command fails, report the failure.
 
 ## Maintain rules
 

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

@@ -0,0 +1,16 @@
+# Let rules
+
+## Prefer explicit dependencies over shared convenience bindings
+
+### Rule
+
+Do not add a `let` binding only to let sibling values share one expression.
+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.
+
+### Rationale
+
+Shared convenience bindings hide dependency direction.
+Explicit references show which value owns a concept and which value consumes it.
+