Markdown link checker CLI tool

Build a dependency-free Node.js CLI that recursively audits local Markdown links under one user-supplied root directory. Read .md files only, skip .git, node_modules and symlinks. Check inline links and images plus reference-style link definitions. Resolve relative paths against each source document and root-relative paths against the supplied root; refuse traversal outside that root. Treat http(s), mailto and data links as external and never fetch them. Decode percent-encoded local paths; check file existence and, for .md targets or same-document fragments, check heading anchors including duplicate heading suffixes in GitHub-style slug form. Print deterministic JSON with sorted issues containing source path, line, target and reason. Exit 0 if clean, 1 if broken links, 2 for invalid invocation or root. Never modify scanned files. Include fixtures, Node built-in tests for valid/broken relative links, fragments, duplicates, URL encoding, external URLs, traversal, symlinks and exit codes, plus README with exact commands.