Exclude node_modules from linter

The linter scans all markdown files and fails on files inside
node_modules. Exclude the directory like other non-content paths.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

lib/linter.rb

@@ -18,7 +18,8 @@ class Linter
     %r{\Aadmin/index\.md},
     %r{\A[^/]*/examples/},
     %r{\A_includes/},
-    %r{\Atest/}
+    %r{\Atest/},
+    %r{\Anode_modules/}
   ].freeze
 
   WHITESPACE_EXCLUSIONS = [