Add VitePress website with developer landing page and GitHub Pages deploy

[RyanLee-Dev]

Summary

Adds a VitePress website (developer landing page + documentation) published to GitHub Pages.

• Docs stay where they are. The content root is the repository root; docs/** and contracts/** keep their paths as URLs. No file under docs/ or contracts/ changes. The sidebar, the landing page's documentation index and /llms.txt are generated from docs.json at build time, so new pages flow in automatically. Each page's source is also served as .md, and the README paths in docs.json redirect to their section index. Relative links to repository-only files (CONTRIBUTING.md, openapi.yaml, …) are rewritten to GitHub at build time.
• Landing page (EN /, ZH /zh/). Geeky terminal style: an animated ASCII rendering of the logo (decode-in, scan line, pointer-reactive, pauses off screen, honours reduced motion), an interactive Session builder that generates the real SDK call and shows Core's accept/reject verdict per harness/protocol, boundary → protocol doc map with animated packets, ASCII diagrams for the Session rules, a typing terminal for install, and an ASCII "ONE CORE. MANY AGENTS." outro. It reuses architecture.png, development-architecture.png, architecture-api-surfaces.png and the EN/ZH console screenshots from docs/assets. Content follows README, quickstart, self-hosted, model-execution and the project write-up.
• Deployment. .github/workflows/website.yml builds and tests on PRs that touch website inputs and deploys main to Pages. The build reads the Pages base path, so it works at /<repo>/ and on a custom domain. Registered in scripts/ci_plan.py as a lint-only workflow.
• Tests. website/tests: navigation derivation, build output (every docs.json page has HTML + .md, redirects, llms.txt), and the landing harness table checked against internal/harnessconfig/builtin/catalog.json and contracts/agents-api/model-execution.md.

Needs a maintainer

• Enable Pages once: Settings → Pages → Source: GitHub Actions (optionally set a custom domain there).
• Mintlify coexists. docs.json and docs/development.md still describe the Mintlify site; this PR leaves docs/ untouched by request. If VitePress replaces Mintlify, a follow-up should update docs/development.md and remove the Mintlify-only fields/.mintignore.

Verification

• pnpm --filter @oac/website build and test (12 pass); build also run with WEBSITE_BASE=/OpenAgentCore and every internal link, asset and redirect checked in a browser (0 broken).
• make check-names check-docs check-ci, name guard with the new files tracked, actionlint 1.7.12.
• Playwright screenshots at 1440px (dark/light, EN/ZH) and 390px (no horizontal overflow); content visible with JavaScript disabled; no console errors.
• Independent review pass; its findings (name guard, mcode token limits, overclaims, deploy cancellation, Pages permission) are addressed.

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Project	Status	Preview	Updated
openagentcore	🟢 Ready	View Preview	Oct 1, 2026, 9:56 AM

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