CI guard: flag relative links on index.md/index.mdx pages in PRs

[digitaldiina]

Summary

Add a CI check that flags relative links (./..., ../...) authored on index.md/index.mdx files in a PR diff. This is the exact bug class fixed in #375/PR #376 and tracked at larger scope in #377: any page built from a literal index.md/index.mdx file serves at its folder's own bare path with no trailing slash, so a relative link written on it resolves against the wrong parent folder in production. No existing static tool catches this, since both mint validate (doesn't check links at all) and this repo's own tools/triage-broken-links.py resolve relative links via filesystem-relative path math against the source file's disk location, not the live URL the browser actually resolves. The target genuinely exists on disk, so today's checks pass while the live link still 404s.

What to flag

On any changed index.md/index.mdx file in a PR:

• A relative markdown link target starting with ./ or ../, inline ([text](./foo)) or reference-style ([n]: ./foo).
• Same for raw HTML href values (<a href="./foo">, <Card href="../foo">), per the same rule documented in contribute/markdown-guide/links-in-docs.mdx.

What NOT to flag

• MDX import statements (import Foo from './foo.mdx') — these are compile-time component imports, not page navigation links, and are unaffected by this bug.
• Anything inside a fenced code block (```) or inline code span (`...`) — sample/documentation text, not a real link.
• Links on non-index files (the bug is specific to index.md/index.mdx, confirmed in Relative links on release-notes index pages resolve to wrong URL (404) #375 — leaf pages resolve .//../ links correctly).
• Already-root-relative (/...) or absolute (https://...) links.

Suggested shape

Model on the existing tools/ci/*-guard.py + .github/workflows/*-guard.yml family (e.g. version-frontmatter-guard.yml, landing-page-template-guard.yml) — warn or fail on the PR, don't auto-fix (unlike the auto-fix-workflow family, which has its own documented race-condition problem when multiple such workflows fire on the same push).

Related

• Relative links on release-notes index pages resolve to wrong URL (404) #375 (root cause + release-notes/ fix)
• Relative links on index pages outside release-notes/ resolve to wrong URL (404) #377 (same bug, remaining scope outside release-notes/)
• CI guard: flag renamed/moved/deleted pages missing a redirects.json entry #340 (existing, separate CI-guard issue for missing redirects.json entries on renamed/moved/deleted pages — different check, same "no CI models the live site" theme)