Restore AI-era title and simplify MCP setup

[Chenglong-MS]

Summary

• Restore the public Flint title to Flint: A Visualization Language for the AI Era.
• Apply the title consistently in the README, landing hero, and browser tab title.
• Keep the original MCP intro, then add a TL;DR setup request users can paste into their agent.
• Add a top-right copy control for the TL;DR setup prompt.
• Place install/GitHub links inline with the TL;DR text, without arrows; the install link scrolls to the page section directly.

Validation

• npm run build -w site
• git diff --check
• VS Code diagnostics on touched files

Notes

• This is site/docs copy only. It does not bump package versions or create a release patch.