Verify external source URLs in content/*.md are reachable. HEAD request
with GET fallback, treats known bot-blocker 403s (hhs.gov, jamanetwork,
consilium.europa.eu etc.) as OK.
Usage:
scripts/check-links.sh # all content files
scripts/check-links.sh --fast # stop at first failure
scripts/check-links.sh --verbose
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>