Update boilerplate skills link on boilerplate overview page

[bdenham]

Purpose of this pull request

One-line link update on the boilerplate overview page so it points to the renamed boilerplate-skills page instead of the old ai-agent-skills slug.

Associated JIRA ticket

N/A

Staging preview

N/A

Affected pages

• src/content/docs/boilerplate/index.mdx

Summary

Split out of #953. Merge order: this PR must merge after #953, not before. It links to /ai/boilerplate-skills/, which doesn't exist on release until #953 creates it — #953's redirect only covers the old slug redirecting to the new one, not the reverse, so merging this first would ship a live 404.

Testing

Verified the link target matches the renamed page in #953.

[bdenham]

Closing — this fix is folded back into #953. The starlight-links-validator build check validates links against the branch's own content, not the redirects config, so a PR that deletes/renames a page must update its inbound links in the same PR or the build fails. Splitting this into a separate PR would have broken #953's own build.