feat: add public demo site

[shaypal5]

Summary

• add an explicit public demo build mode that uses bundled sample data and does not call operator APIs
• configure Vite base-path support for GitHub Pages deployment under /agent-comms-demo/
• add a top-of-README screenshot linking to the live public demo
• fix app-shell text color inheritance so night mode applies consistently to top-level text

Live demo

• Demo site: https://agent-comms.github.io/agent-comms-demo/
• Demo repository: https://github.com/agent-comms/agent-comms-demo

Validation

• npm run check
• npm run test
• npm run build
• npm run build:demo
• git diff --check
• live screenshot capture with npx --yes playwright@latest screenshot --wait-for-timeout=1000 --viewport-size=1440,1040 'https://agent-comms.github.io/agent-comms-demo/?verify=cli' /tmp/agent-comms-demo-live.png