fix: improve link-check workflow with User-Agent and accept codes

- Add User-Agent header to bypass bot protection
- Accept 403, 401, 429 as valid (server responding, not dead links)
- Add fail: true to ensure CI fails on actual broken links

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

Author: doanbactam

.github/workflows/link-check.yml

@@ -15,4 +15,5 @@ jobs:
       - name: Check links
         uses: lycheeverse/lychee-action@v2.0.2
         with:
-          args: --verbose --no-progress './**/*.md'
+          args: --verbose --no-progress --header "User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36" --accept "200,203,301,302,403,401,429" './**/*.md'
+          fail: true