docs: Rewrite the README for Readability

[Erick Jaimez (ejaimez14)]

What

• Rewrites the README so it reads the same way as the Claude Code and Codex ones
• Replaces the install steps, which pointed at a Settings page Cursor no longer has
• Says a Mac needs nothing installed, now that the plugin carries its own build

Why

• Anyone following the old instructions could not find the screen they described

Test Plan

• Every setting, default and path checked against the source rather than carried over
• Install steps taken from the current Cursor app and its documentation