infra(landing): provision Vercel project + apex DNS for agentskit.io

[EmersonBraun]

Context

PR #766 merged `apps/landing` (closes #230) — the standalone Next.js 16 marketing site. The app builds and deploys cleanly but has not yet been wired to its own Vercel project or to the apex domain. The existing `agentskit-doc` Vercel project still serves `www.agentskit.io`, so today the landing PRs render only on PR previews.

What to ship

1. Create a new Vercel project `agentskit-landing` (separate from `agentskit-doc`).
   • Root directory: `apps/landing`
   • Build command: inherits from `apps/landing/vercel.json` (`pnpm --filter @agentskit/landing build`)
   • Install command: `pnpm install --frozen-lockfile`
   • Framework preset: Next.js
2. Plan the apex / subdomain split:
   • `agentskit.io` apex → landing
   • `docs.agentskit.io` → existing docs project (currently `www.agentskit.io`)
   • `www.agentskit.io` 301 → `agentskit.io`
3. DNS migration:
   • Update `agentskit.io` apex A record (or ALIAS / CNAME flattening) to Vercel
   • Add `docs.agentskit.io` CNAME to the docs project
   • Verify SSL provisioning and HSTS preload eligibility
4. After cutover, update `apps/landing/app/_components/links.ts` so `LINKS.docs` points to `https://docs.agentskit.io\` instead of the current `www.agentskit.io/docs` path.
5. Smoke test:
   • All landing CTAs land on a real docs page (no 404s)
   • Old GitHub Pages URL (`emersonbraun.github.io/agentskit`) redirects to apex

Acceptance

• `agentskit.io` resolves to the landing app, SSL valid
• `docs.agentskit.io` resolves to Fumadocs, SSL valid
• No broken links from landing → docs
• Old domain redirects in place

[EmersonBraun]

This task needs Vercel + DNS provider credentials that an automated coding agent doesn't hold. Needs to be done by a maintainer with org access. Leaving open.