feat(vercel): add Vercel Marketplace onboarding skill

[LorrisSaintGenez]

What does this skill do?

Add algolia-vercel, a standalone skill that provisions and repairs Algolia through the Vercel Marketplace, then validates a first search. This PR only adds the new skill — no existing skill is modified.

Follow the prioritized rules and correct/incorrect examples used by Vercel’s popular skills.

Checklist

• python3 scripts/validate_skills.py . passes (0 errors)
• Skill description is under 1024 chars and includes WHEN and WHEN NOT to trigger
• evals/evals.json exists with at least 1 realistic prompt
• Skill is self-contained — no ../other-skill/ references
• Registered in .claude-plugin/marketplace.json (new skills only)
• Goes in skills/ here if useful to customers; internal-only skills → algolia/internal-skills

Manual verification

1. From this branch, run npx skills add . --skill algolia-vercel.
2. Ask an agent, plan only: “Use the algolia-vercel skill to set up Algolia through the Vercel Marketplace.” Expect listing existing resources before provisioning, and matching connection/pull environments.
3. Ask: “Use the algolia-vercel skill to repair missing local credentials for an existing Marketplace Algolia.” Expect no duplicate resource and no demo import.
4. Ask for a search UI using existing standalone credentials, without naming the skill. This is a scope check on the skill’s WHEN NOT section — the skill should not apply, so no Marketplace provisioning is in scope.

CLI references and local credential handling checked. No live provisioning or deployment performed.

After merge

Associating algolia-vercel with Algolia’s Vercel Marketplace listing is a follow-up I’ll coordinate separately; it isn’t a dependency of this repo change.