feat: add markdown-link-check action

[blink1073]

References

Description

Adds a new reusable composite action, markdown-link-check, which checks the links in every .md file using markdown-link-check. Dead links fail the job.

Bundled defaults ignore ^https://github.com and ^https://nbviewer.org/ links, retry on HTTP 429, and treat 200, 206, and 403 as alive. A repo-root .markdown_link_config.json is merged with those defaults: its top-level keys override the defaults, while ignorePatterns is the union of the default and user patterns (defaults first, duplicates removed). A missing config file is fine, and the default ignore patterns cannot be removed.

Changes

• actions/markdown-link-check/merge_config.py — stdlib helper that merges the user config with the bundled defaults and validates it.
• actions/markdown-link-check/action.yml — composite action that merges the config, installs markdown-link-check@<version>, and checks all .md files. Inputs: version (default 3.15.0), folder-path (default .), ignore-paths (default node_modules,.git), and use-quiet-mode (default no).
• actions/markdown-link-check/test_merge_config.sh — 9 test cases covering the defaults, union, de-duplication, overrides, and validation failures.
• .github/workflows/tests.yml — adds a Test merge_config.py step to the test_scripts job and a dogfooding test_markdown_link_check job (added to tests_check.needs).
• README.md and CLAUDE.md — document the new action.

Backwards-incompatible changes

None

Testing

• bash actions/markdown-link-check/test_merge_config.sh — 9/9 passing.
• python3 -c "import yaml; yaml.safe_load(...)" validates action.yml and .github/workflows/tests.yml.

just lint-all could not be run locally because Poetry/pre-commit are not installed in this environment; git diff --check reports no whitespace errors.

AI usage

• Some or all of the content of this PR was generated by AI.
• The human author has carefully reviewed this PR and run this code.
• AI tools and models used: opencode (deepseek-v4p1-flash)

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The changes since the previous review appear safe to merge.

Summary

Adds markdown-link-check with bundled defaults, support for a repository config, and configurable scanning inputs.

• Moves version into a quoted environment variable.
• Rejects missing or non-string pattern values and adds regression tests.
• Adds local fixtures for nested files, ignored folders, dead links, and clean scans.
• The previous threads are unnumbered, so they have no entries in previousFindings. The version and pattern findings are addressed. The scanning fixtures cover the scenarios requested by the testing finding.
• No new actionable issues were established. Tests were not run during this review.

_{Reviews (2) · Last reviewed commit: "chore: silence zizmor adhoc-packages not..." · Reviewed by Greptile}