[codex] add SpecGov README and GitHub Pages site

[paladini]

Summary

• Rewrites the README as a complete global adoption guide for SpecGov, including source installation, CLI usage, manifest design, GitHub Action setup, adoption recipes, reports, and security posture.
• Adds a static GitHub Pages site under docs/ with responsive product documentation, a visual report asset, copy/select helpers for code blocks, and links to examples.
• Adds a Pages deployment workflow and updates SpecGov's own manifest/spec docs so the new documentation surface is governed.

Validation

• npm test
• npm run build
• npm run lint
• npm run typecheck
• npm run format:check
• node dist/cli.js check-pr ... in advisory mode: pass, 0 findings
• node dist/cli.js check-pr --mode strict ...: pass, 0 findings
• Browser QA on http://127.0.0.1:4173/ for desktop and mobile: page identity, nonblank render, no console warnings/errors, visual asset loaded, no horizontal overflow, CTA visible, first-fold hint visible, and copy fallback interaction verified.

Notes

The README and Action examples use paladini/specgov@main because the repository currently has no version tag or release. After the first tag exists, the docs should pin the Action example to that tag.