Add a repository Markdown link check to CI

[pranavdhawann]

Context

Release verification currently uses an ad hoc local relative-link scan, so later documentation changes can merge with broken internal links.

Expected behavior

CI runs a deterministic link check for repository-relative Markdown links and avoids false positives on badges or external URLs.

Likely files

.github/workflows/ci.yml, package.json, scripts/ or a small checked-in config

Acceptance criteria

• Check README, root community files, and docs/**/*.md.
• Fail on a missing local target.
• Ignore anchors and external URLs or check them in a separate non-blocking lane.
• Document the local command in CONTRIBUTING.md.