docs: rewrite the README around the desktop app and simplify the badges

Lead with the desktop download, expand the agent capability sections, and cut
the README to the essentials. Replace the heavy for-the-badge row with a flat
row: release, desktop downloads, license, macOS/Windows, Node.js, visitors.
Use the desktop screenshot as the hero image and point every download link at
code.pythinker.com.

Author: elkaix