add Open Design plugin landing page

[joeylee12629-star]

Why

Open Design needs an official, installation-focused landing page for its Codex/ChatGPT plugin. The current marketing site does not give users a direct path from discovery to a copyable install instruction, nor does it show the real plugin workflow inside Codex.

This adds the page requested for the website launch and keeps the experience consistent with the existing Open Design marketing surface.

What users will see

• A new localized /open-design-pugin/ landing page focused on installing Open Design from a single copyable instruction.
• Real Codex/ChatGPT screenshots covering installation, prompting, the live handoff, and the finished result.
• Four compact example-and-prompt pairs, plus locale-aware navigation in 11 supported languages.
• New entry points in the Product dropdown and Product footer column.

Surface area

• UI — new marketing page and navigation/footer entry in apps/landing-page
• Keyboard shortcut — new or changed
• CLI / env var — new od subcommand or flag, new tools-dev / tools-pack flag, or new OD_* env var
• API / contract — new /api/* endpoint, new SSE event, or changed shape in packages/contracts
• Extension point — new entry under skills/, design-systems/, design-templates/, or craft/, or change to the skills protocol
• i18n keys — added localized page copy for en, zh, ja, ko, de, fr, ru, es, pt-br, it, and tr
• New top-level dependency — adding any new entry to the root package.json
• Default behavior change — changes what existing users experience without opting in
• None — internal refactor, docs, tests, or translation update only

Screenshots

• Cloudflare Pages preview
• Chinese preview

Bug fix verification

• Not a bug fix; this is a new marketing surface. Visual behavior was verified in the running Astro site at desktop width, including the Chinese ChatGPT subtitle alignment and responsive wrapping.

Validation

• pnpm guard
• pnpm typecheck
• pnpm --filter @open-design/landing-page typecheck
• pnpm --filter @open-design/landing-page build:static — 5,258 pages built
• Verified all 11 locale routes return HTTP 200 in the local preview
• Verified the deployed English and Chinese preview routes return HTTP 200
• Browser-checked the English and Chinese landing pages against the final rendered UI

Production gate

This PR intentionally deploys the new route to staging first. Do not promote the page and its global navigation entries to production until all of the following are true:

• Open Design 0.17.0 or a newer compatible runtime is publicly released.
• nexu-io/open-design-agent-plugins/release-manifest.json no longer reports distributionStatus: unreleased-candidate or a pending compatible runtime.
• https://open-design.ai/codex-plugin returns HTTP 200 after promotion, and the copied instruction completes a clean installation against the published releases.

Current verified pre-release state (2026-07-30): production short URL returns 404; PR preview returns 200; latest public Open Design release is open-design-v0.16.1; plugin manifest requires 0.17.0. These are release-ordering gates, not blockers for merging the staging-only page implementation.

[github-actions]

🚀 Landing page preview

This PR is deployed to a Cloudflare Pages preview — not staging or production:

• Stable alias: https://pr-6241.open-design-landing-staging.pages.dev
• This build: https://b9e49501.open-design-landing-staging.pages.dev
  Updated for commit 70f4a2d.

[mrcfps]

@joeylee12629-star Thank you for the thoughtful localized landing page and the real end-to-end workflow screenshots — the route, responsive presentation, locale variants, and copy interaction are all put together carefully. I found one launch-readiness mismatch in the primary installation path; I’ve left the concrete release-gating options inline. Addressing it before production promotion will keep the first-run experience aligned with the promise of the page. 🙏

_{🔁 Powered by Looper · runner=reviewer · agent=codex · An autonomous AI dev team for your GitHub repos.}

[mrcfps]

@joeylee12629-star Thank you for the thoughtful landing-page work and the careful localization pass. I verified the changed routes, responsive layout, screenshots, navigation entry points, copy interaction, static build, and current CI results. I found one remaining launch-readiness issue in the primary copied instruction; since this branch deploys to staging and production promotion is manual, I’m leaving it as a non-blocking follow-up, but it should be closed before the page goes live. 🙏

_{🔁 Powered by Looper · runner=reviewer · agent=codex · An autonomous AI dev team for your GitHub repos.}

[joeylee12629-star]

@mrcfps All review threads are resolved, the production-release dependency is documented as an explicit PR-body gate, CI is green, and the branch is conflict-free. Please complete the final review when ready so this staging-only implementation can merge.