fix(validate): skip upstream/ and walkthrough/ in markdown filename check

The markdown-filenames validator was scanning into upstream/meander/
(a git submodule whose file naming we don't control) and blocking
pnpm check with violations on AGENTS.md and test-walkthrough-docs/.

Also proactively skip walkthrough/ — the tour build output is
generated HTML, not hand-authored markdown, so there's no reason
to crawl it either.

Matches the same SKIP_DIRS treatment in scripts/validate/no-cdn-refs.mts.

Author: jdalton

scripts/validate/markdown-filenames.mts

@@ -61,6 +61,10 @@ const SKIP_DIRS = new Set([
   '.nuxt',
   '.output',
   'reports',
+  // Vendored submodule — we don't control filenames in upstream code.
+  'upstream',
+  // Tour build output — generated HTML, not hand-authored markdown.
+  'walkthrough',
 ])
 
 type MarkdownFilenameViolation = {