Restore automated link validation

[bvolpato]

Summary

The catalogue had eight broken link references, while the checks on #163 and #164 remained queued without jobs. This integrates both reviewed PR commits with the remaining link cleanup and a fresh run of the required checks.

Changes

• Preserve the original commits from Delete .gitlab-ci.yml #163 and Delete archived ChatGPT Auto-Continue #164, removing the obsolete GitLab reverse-sync configuration and archived ChatGPT Auto-Continue entry.
• Remove Fandom Focus and its empty category. Its install, catalogue, feedback, and screenshot URLs return 404.
• Run Lychee on PRs, master pushes, and daily at 06:17 UTC without a persistent cache. Keep the required lychee job name.
• Retry a failed scan once after 15 seconds, cap the job at 15 minutes, and retain reports for 14 days.
• Share local and CI settings in .lychee.toml and document how to run the check. Existing exclusions and HTTP 403/429 handling remain explicit; these responses need manual verification.

Testing

• Clean Node 24 install, full ESLint check, and dependency audit passed; zero vulnerabilities.
• Actionlint and git diff --check passed.
• Lychee 0.24.2 scanned 692 references across 411 unique links with zero errors.
• Local HTTP probes confirmed valid anchors pass, a transient 503 recovers on a fresh scan, and a persistent 404 still fails after retry.

Merge with a merge commit to preserve the original PR commits. Hosted checks must pass before merging.

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