docs(skills): add nix let review rule

.pi/prompts/review-nix.md

@@ -13,9 +13,19 @@ Do not report preferences unless a loaded rule states them.
 Use this format for each finding:
 
 ```text
-`path:line`: Finding title.
+`code-path:line`
+> `smallest reviewed-code excerpt that shows the violation`
+
+`rule-file:line`: Finding title.
 Summary: why the code breaks the rule
 Fix: suggested fix
 ```
 
+Keep the code excerpt to one to three lines when possible.
+Use a longer excerpt only when the violation needs surrounding context.
+Use `...` to omit unrelated lines from a multi-line excerpt.
+Use a skill-relative rule citation, such as `let.md:7`.
+Make the fix direct.
+Do not include alternatives when one fix best follows the cited rule.
+
 It is acceptable to report no findings when the reviewed Nix already follows the loaded rules.