Prepare local-first public release and remove Fly deployment path

[Obsecurus]

Summary

• pivot repo messaging to engineer-focused, local-first public release
• remove Fly deployment artifacts and stale rollout docs
• rewrite GitHub Pages site content to emphasize free usage ($0 core workflow)
• update docs for GitHub Pages + Namecheap DNS automation
• replace Fly-specific UI/server wording with neutral manual-hosting wording
• keep tests aligned with new site/repo posture

Removed

• .github/workflows/fly-deploy.yml
• fly.toml
• Dockerfile.fly
• interview-app/fly.toml.example
• obsolete docs: Fly/distribution/deploy-queue/family-launch
• site/app.js (old browser demo script)

Validation

• cd interview-app && node --check server.js builder.js mechanic.js safety.js
• cd interview-app && AUTO_GENERATE_CERTS=false node --test test.js

All tests passing (90/90).