Limit Markdown link checks to newly added blog posts

Summary

Add a rule that excludes existing blog posts from Markdown link checks. Apply Markdown link checks when a pull request adds a new blog post.

Rationale

Historical blog posts can contain stale external links. Those links should not block unrelated pull requests. New blog posts must still meet the Markdown link-check requirement.

Affected areas

• Markdown link-check configuration and CI scripts.
• Blog-content change detection.
• package.json scripts and the validation workflow, if required.

Required behavior

• Detect blog post files that are newly added in the pull request.
• Run Markdown link checks for each newly added blog post.
• Do not run Markdown link checks for blog post files that existed before the pull request.
• Keep Markdown link checks for non-blog content unchanged unless the implementation explicitly documents a required exception.
• Ensure a modification to an existing blog post alone does not cause its historical links to fail the check.

Acceptance criteria

• A pull request that adds a blog post with a broken Markdown link fails the relevant check.
• A pull request that changes an existing blog post does not check unrelated historical links in that post.
• A pull request with no newly added blog posts does not run blog-post Markdown link checks.
• The rule works for pull request and preview CI contexts.
• The implementation includes automated coverage for added and existing blog post cases.

Backlinks

• Pull request: https://github.com/openfga/openfga.dev/pull/1341
• Request: https://github.com/openfga/openfga.dev/pull/1341#issuecomment-5315293613
• Requested by: @SoulPancake

Start with the Markdown link-check configuration, CI scripts, package.json scripts, and validation workflow, then trace how blog-content changes are detected in pull request and preview contexts. Add automated coverage for newly added and existing blog posts, including cases with no new posts; done means only new blog posts are checked while existing and non-blog behavior remains unchanged.