Fix markdownlint errors in Digestible Interfaces chapter

- Update MD024 config to use siblings_only (allows repeated subheadings
  under different parent headings)
- Add 'text' language specifier to error message code blocks

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: testac974

.markdownlint.json

@@ -4,7 +4,7 @@
   "MD003": { "style": "atx" },
   "MD007": { "indent": 2 },
   "MD013": false,
-  "MD024": { "allow_different_nesting": true },
+  "MD024": { "siblings_only": true },
   "MD025": false,
   "MD033": { "allowed_elements": ["br", "details", "summary"] },
   "MD041": false,

book/part1-foundations/04-digestible-interfaces/08-measuring-digestibility.md

@@ -248,7 +248,7 @@ If the answer is "probably not," the interface needs to be more explicit.
 
 **Example prompt**:
 
-```
+```text
 Given this function signature, write code that creates a user:
 
 def create_user(request: CreateUserRequest) -> User:
@@ -298,15 +298,15 @@ Error messages should answer:
 
 **Good error**:
 
-```
+```text
 ValidationError: CreateUserRequest.password must be at least 8 characters.
   Received: "abc" (3 characters)
   Fix: Provide a password with 8 or more characters.
 ```
 
 **Bad error**:
 
-```
+```text
 ValueError: Invalid input
 ```