CI: add a dependency-free checker for repository-local Markdown links

[bitjaru]

Why

StyleSeed has multiple maintained READMEs and engine documents. Relative links can drift during file moves, but CI currently validates product contracts rather than repository-local Markdown targets.

Scope

Add a dependency-free checker at scripts/check-markdown-links.mjs that validates repository-relative file and image targets in:

• README.md, README-KR.md, CONTRIBUTING.md, and ROADMAP.md
• maintained Markdown under engine/ and docs/

This first slice does not need to make network requests or validate remote URLs.

Acceptance criteria

• Strip query strings and #fragment suffixes before checking the local file target.
• Ignore absolute URLs, mailto:, hash-only anchors, and examples inside fenced code blocks.
• Report the source file, line number, and missing target.
• Exit non-zero when a target is missing and zero on the current repository.
• Add focused fixtures or tests for a valid target, missing target, external URL, and fenced example.
• Add the checker to .github/workflows/validate-engine.yml.
• node scripts/check-markdown-links.mjs, the new tests, and git diff --check pass.

Boundary

Do not add an npm dependency or crawl external sites. Keep this PR to deterministic local links.

New contributors are welcome to claim this issue by leaving a comment.

[PrasenKakade21]

Hey, I would like to take on this issue.