Restore markdown link checking

[nightt5879]

close #5414

Changes:

• Restored the Linkspector check and Chrome workaround that were temporarily removed in Remove the failing markdown link checks for now #5413.
• Changed filter_mode to added.

Rationale:
The previous file mode checked the entire modified file, which meant pre-existing broken links or temporary failures on external websites could block otherwise unrelated PRs. Using added limits the check to broken links introduced by newly added lines.

Impact:

• Restores Markdown link validation in CI.
• Reduces false positives and prevents unrelated PRs from being blocked by existing link issues.

Validation:

• actionlint passed.
• npm run validate-markdown passed.
• npm test passed: 8/8.
• git diff --check passed.

Schema:
No schema changes are required.

AI disclosure:
Codex was used to analyze historical CI logs, update the workflow, and run validation checks. I personally reviewed and confirmed the changes.

• no schema changes are needed for this pull request

[lornajane]

Thanks @nightt5879 , just a few comments immediately:

• we do need to check the whole file, not just the added lines, in case of link decay on our specification documents. This was an intentional design choice.
• I'm also switching the status on this to draft, there's work going on in the wider build system that needs to land first (see [DRAFT] Migrate to new shared build infrastructure. #5424) and we may ask for more updates once that is complete.