fix: restore GitHub Pages assets

[paladini]

What changed

• moved the landing-page CSS, JavaScript, and social image back under docs/assets/, which is inside the GitHub Pages publish source
• updated local asset references to portable assets/... paths
• aligned canonical, Open Graph, Twitter, and JSON-LD URLs with https://mcp-me.page/
• added a regression test that rejects local references escaping docs/ or pointing to missing files
• documented the fix under [Unreleased]

Root cause

PR #12 moved the static assets out of docs/, while GitHub Pages is configured to publish only main:/docs. The Pages build succeeded but omitted those files, so CSS, JavaScript, favicon, and the social image returned 404.

Validation

• npm run lint
• npm run typecheck
• npm test - 2,214 tests passed
• npm run build
• local browser smoke test - no console errors; HTML, CSS, JavaScript, and SVG requests returned 200