fix: resolve lychee root-relative link errors

--exclude runs after URL construction; root-relative paths fail
before that stage. Use --root-dir . so lychee builds file:// URLs,
then --exclude 'file://' to skip all local path checks.
External links are still verified. Tested locally: 0 errors.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: greynewell

.github/workflows/ci.yml

@@ -32,5 +32,6 @@ jobs:
             --accept 200,201,202,203,204,206,301,302,303,307,308,429
             --max-concurrency 8
             --timeout 20
-            --exclude '^/'
+            --root-dir .
+            --exclude 'file://'
             README.md **/*.mdx