docs: prepare beta release

[papperrollinggery]

Summary

• Add beta install, source install, MCP setup, local capability source, and smoke test docs.
• Add privacy boundaries and real beta use cases.
• Add release checklist for beta publish and stable promotion.
• Include the new user docs in the npm package allowlist and public audit.

Verification

• npm run build
• npm run lint
• npm test
• npm run audit:public
• npm pack --dry-run --json
• tarball install smoke with empty HOME
• npx gitnexus detect_changes --repo lazy-brain